|I'm certain that Eldians are the descendants of devils. And I'm certain...that we too are devils.|
|— Magath agrees to Willy Tybur's plan|
Formerly a commander (隊長 Taichō?) of the Eldian Unit of the Marley army, and before one of the chief overseers of the first Warrior candidates during their training, he later led the Eldian Warriors in the Marley Mid-East War against the Mid-East Allied Forces, playing a key role in Marley's victory.
Magath is an older man with light eyes and a thin beard, mustache, and short thinning hair. He has visible wrinkles from age but has an overall healthy build and appearance. Like his fellow soldiers, he wears the standard Marleyan soldier uniform consisting of a light jacket and pants with a black clip at the collar, supply packs on the hip, suspender straps, a hard hat with a black stripe around it, and tall combat boots.
Theo Magath is extremely dutiful, and most of his personality and choices revolve around what he believes is his duty as a soldier of Marley. Even though he, as a typical Marleyan, is prejudiced against Eldians, valuing the life of the Marleyans above theirs and being angered when one questions his orders, he is willing to put his opinion aside and let the Eldians under his command speak, in the case they might have something useful to add to the current situation, such as Colt Grice and Zeke Yeager.
He readily accepted Willy Tybur, an Eldian, as his superior after figuring it out, and accepted the fact that he will soon inherit the role as the general of Marley's army more out of a sense of duty to his commanding officer and homeland than personal desire, even though Willy had presented it as an offer instead of an order. He even states that the army does not belong to him, but the nation itself.
Many of his decisions make him appear as though he considers the Eldians under him as expendable pawns and uncaring towards their lives, such as deciding to sacrifice 800 Eldian soldiers to destroy the anti-Titan artillery, while keeping two holders of the power of the Titans and the Warrior candidates safe. However, he mostly goes with this course of action because it is most beneficial to Marley, as he wanted to keep Warriors, as well as the Warrior candidates, out of harm's way since most of his nation's might comes from the Titans, being aware of the other nations' technological superiority. Compared to other Marleyans, he is quite tolerant of Eldians, willing to hear them speak, does not appear to get physical with them, and even shows genuine concern not present in other Marleyans, noted when wishing a safe return to the Warriors who were leaving for the Paradis Island Operation.
Magath proves himself to be very cunning and perceptive, managing to figure out the Tybur family was secretly ruling Marley, and seeing through the country's deceptions in its propaganda. He also speaks with a certain level of dry wit, noting to Willy Tybur that the statue of Helos was the true spirit of Marley, due to being completely hollow.
While the details of his rise to power are unknown, Magath has been a member of the Marley military for over twenty years. Throughout this time, he acted as one of the chief overseers of the Warrior program, evaluating the performances of each individual Warrior candidate and selecting the top performers to be inheritors of the power of the Titans. By the early 840s, Magath had achieved the rank of commander.
One day during the early 830s, Magath was supervising a training session of the Warrior cadets with Tom Xaver, the Beast Titan of that time. While the two were watching the cadets training, Magath noticed a cadet trailing behind the others and then commented that this Warrior in particular, Zeke Yeager, did not seem to have the guts the become a Warrior.
Magath was then seen belittling Zeke in front of all the other cadets, telling him that Marley needed real men as Warriors, and that they would never give the nation's precious Titans to an Eldian like him.
Time passes by, and during a public training where the parents of the Warrior cadets were able to see their kid's performances, Magath noticed Zeke and belittled him one more time, telling him that he did not have a place in the training and that Marley did not need him.
In the year 843, Magath was present during the final battles between Marley and an enemy nation, witnessing their destruction at the hands of newly trained Titan Warriors. He admitted that the Warriors raised from childhood were more effective than their predecessors, but doubted the Marleyan army's decision to entrust the upcoming Paradis Island Operation to children. Two years later, he debriefed the Warriors on the logistics of the plan and departed with those chosen for the mission to the border of Paradis Island. He gave his final debriefing to the four Warriors before leaving them, hoping they would all return alive with the Founding Titan.
At the climax of the Marley Mid-East War, Commander Magath leads an 800-Warrior unit against the Mid-East Allied Forces at the battle of Fort Slava. When the Warriors' trench line could advance no further towards the bunkers around the fort, candidate Colt Grice suggests to Magath that their Jaw Titan and Cart Titan should be used against the Allies. Magath refuses this suggestion, pointing out the Allies' anti-Titan artillery and the great risk of losing their Titans against it. Candidate Gabi offers to use a bundle of grenades to destroy the armored train carrying the artillery, but Magath refuses to waste an Eldian soldier needlessly. Gabi becomes sarcastically coy with the commander, wondering why her life is more important than the 800 others in the unit, and Magath eventually gives in and sends her out. Once Gabi succeeds in destroying the armored train, Magath sends out Porco Galliard, the Jaw Titan, to attack the fort.
As Galliard's Jaw Titan and Pieck's Cart Titan lead a charge against Fort Slava's defenses, Magath remains behind with Colt and reviews their strategy. Colt correctly guesses that the Warriors should retreat and block escape routes while Zeke's airborne attack continues the assault. When the time comes for Zeke to make use of his Beast Titan to rain mindless Titans down upon the fort, Commander Magath, and the Warriors watch from a distance as Marley's Titans complete the assault on the fort.
Some time afterward, the leaders of the Marleyan military convene to review the world's reaction to the end of the war between Marley and the Mid-East Allied Forces. Commander Magath explains to the outraged general of the military that the technological progress of the world is overcoming the power of the Titans. Magath states that Marley has been too reliant on the power of the Titans, leading to an inferior navy which barely survived the war. In the meantime, other nations have been hard at work developing weaponry to overpower the Titans. Magath proposes that with the progress being made with aerial weaponry, Titans may become useless in future wars. Zeke proposes that Marley must acquire the Founding Titan to gain full mastery over the Titans as soon as possible while focusing on developing conventional weaponry, an idea which impresses the general.
After the meeting, Magath meets with Zeke and Colt, congratulating Zeke on his conduct during the mission. He takes the opportunity to inform Zeke that over the past three years, a total of 32 scouting ships sent to Paradis Island have disappeared without a trace, and he asks for Zeke's opinion on the matter.
Zeke recalls that four Titans, the Attack Titan, the Founding Titan, the Colossus Titan, and the Female Titan, are still located on the island. He proposes that at least two Titans, one of them being Eren Yeager's Attack Titan, have sunk their ships. Magath suggests that battleship support will be necessary for an assault on the island, but Zeke informs him of the existence of anti-Titan weapons on the island, as well as the Ackerman clan which poses a serious threat to their future efforts.
Magath later departs on a train towards Liberio. Despite the Eldians at the back of the train making a loud ruckus, Magath allows them to continue with the celebrations, noting it is only for a single night. When the train reaches the Warriors' hometown, Magath gets off the train with the others and escorts the Eldians into the internment zone.
Whilst the Eldian Warriors hold a private, unofficial meeting, Magath is a part of a team tapping the meeting to test the loyalty of the Warriors. He is slightly disappointed by Zeke's comment that insinuated the meeting was being tapped.
Magath is paid a visit by the Tybur family and is greeted by the family's current head, Willy Tybur. Speaking privately, Willy asks Magath if he is able to tell which Tybur is currently in possession of the War Hammer Titan, but Magath insists he cannot. Magath notes that his leader at headquarters left earlier that day after receiving orders from his superior, and because of that, Magath is now speaking to Willy in this place. He asks if the timing is truly unrelated.
Willy declines to answer, claiming that he came to see the statue Helos, the Marleyan hero who slew the Devil of All Earth. He praises the figure as the spirit of Marley and Magath agrees, remarking that the statue is hollow. Magath intends to reinstate conscription for Marleyans, so war is something more to them than something they read in the newspaper, though he doubts it will be enough to stop Marley's self-destructive march to war. Under the assumption he is speaking to the true power behind Marley, Magath tells Willy that it is too late.
Noting Magath's insinuation, Willy reveals that the Tybur family is, as Magath suspects, the true ruler Marley. However, he insists that Marley's penchant for war has always existed, and that the Tyburs merely allowed the nation to do as they wished as penance for their suffering under Eldian rule. With the passing of time and the obsolescence of the power of the Titans, Willy is concerned about Paradis Island. Commenting that Marley needs another Helos, Willy entreats Magath to work with him. Shortly afterward, Magath is chosen to become the next general of the Marleyan military when the time comes.
One month later, Magath is present at a meeting where the Warriors discuss potential Paradis Island invasion strategies with their Marleyan superiors, though the meeting does not end well. Later that day, he meets with Willy Tybur and hands him a notebook explaining the state of his "house," which Willy notes is in need of "demolition." Willy congratulates Magath on his inevitable upcoming promotion, and the two discuss the military and their country. Magath notices Willy's trembling hands as the latter expresses the burden that comes with his position of power. Changing the subject, Magath informs Willy that some "posts" in his house were still usable, and he has come to learn that "mice" have invaded his home.
As Magath and Willy head to the festival, they discuss the security involved in order to protect Willy and the officials present. Magath does not think it will be possible to defend everyone given that Willy wishes to be on stage and to bait their enemies with as many military officers as possible. Though Willy says Magath can limit it to the useless ones, leaving him free to rebuild the military with whoever he likes, Magath is concerned about the number of people who will die. This is not war like he is used to. They do not know their enemies, how they will attack, and there will be massive crowds of people.
He tells Willy that if he goes ahead with this, he will die, and he is too important to lose. However, Willy confides that if he escapes and is not among those sacrificed, the world will not listen to what he has to say. Magath agrees to go along with the plan, admitting that Eldians are devils, but so are he and Willy.
On the night of Willy's production, Magath brings an armed team atop a nearby building to observe the crowd for any suspicious actions. He takes a report and realizes that the soldiers who have gone to retrieve the Warriors have disappeared and the Warriors are missing as well. Magath has the reserves mobilized to search for them and notes that the event has begun. When the Attack Titan suddenly bursts out of the stage and murders Willy Tybur, Magath watches in shock as it begins a mass slaughter of the Marleyan military's top brass.
The War Hammer Titan emerges to combat the Attack Titan, but is quickly overpowered while Magath's subordinates rush to his side and remind him of their dire situation. Paying little heed to their reports, Magath fires his rifle at the Attack Titan to commence Marley's counterattack against the invader. Noting that their task to defeat the Paradis Island Eldians has been made much easier, he orders his men to head to their battle stations. The War Hammer soon regains the advantage and impales the Attack Titan on a massive pike of hardened flesh, though Magath reminds his soldiers that he dealt the first blow to their enemy.
During the battle between the two Titans, Magath's troops fire a barrage of anti-Titan artillery at the Attack Titan. Koslow worries that they will lose the Founder if they kill the Attack Titan now, but Magath insists that Marley's goal to take the Founding Titan has no place in their policies anymore. The age of the Titans is coming to an end, and ridding Paradis Island of the Founding Titan will deal their enemies a crippling blow, and Magath notes that the War Hammer Titan appears to hold a similar disinterest in taking the Founder.
The War Hammer Titan quickly overpowers the Attack Titan, forcing Eren Yeager to reveal himself. Magath and the others watch as the War Hammer prepares to deal the killing blow, when suddenly Eren calls for Mikasa to finish off the War Hammer. She strikes the War Hammer from behind with eight explosive rods, destroying the War Hammer's nape. As Magath is caught off-guard by the maneuver, other soldiers land on the rooftop and begin a firefight with the Marleyan troops.
Magath and his troops escape the firefight into the shelter of a civilian building where Magath alerts all the nearby army and navy forces, ordering for the Liberio internment zone to be closed off. Magath notes to himself that just as Willy Tybur had planned, all the world would soon join the war against Eldia, though he wonders why the Paradis Island troops are proceeding with their plans knowing of this as well. His thoughts are interrupted when a bomb is sent into their room, forcing them to relocate.
Commander Magath and his soldiers relocate to the plaza where the Attack Titan locates the true "body" of the War Hammer Titan: a crystal encasing the younger sister of Willy deep beneath the ruins of the stage. He watches helplessly as the War Hammer is about to be eaten until Galliard's Jaw Titan arrives at the scene, attempting to bite into the Attack Titan's nape to take the Founder until the Ackerman cuts his jaw muscles. Magath and his troops remain hidden as the Cart Titan and Beast Titan arrive at the plaza to battle the Survey Corps. While the Survey Corps and Warriors engage, Magath is joined by Falco Grice. He initially tells the candidate to escape, but is told that Reiner is unconscious at the back of the plaza. Gabi also joins the Marleyan troops, while Falco states that Eren Yeager was responsible.
At the nearby harbor, there is an explosive transformation that destroys the docking fleet. Magath orders his troops to get away, knowing that only the Colossus Titan could be culpable, but the shock-wave from the blast throws the group to the floor. After the Ackerman takes down the Beast Titan, Magath and the others watch in horror as the soldier drops an explosive into its nape, staring at them all the while. As the attack turns to the Cart Titan, Magath grabs onto Gabi when Falco runs to the Titan's side, begging the Corps not to shoot.
As he and other Marleyan soldiers repel the Corps, Magath orders Gabi to assist Falco in rescuing Pieck, who had come out of her Titan's body unconscious. They later take refuge inside a building, and Magath and the others witness the arrival of an airship coming to aid the Survey Corps. After the airship escapes, he returns to check on Pieck who then explains her suspicions of the mysterious soldier who trapped her and Galliard.
As Marley recovers from the battle, Magath oversees cleanup operations where he has the remains of the Beast Titan investigated. Zeke's limbs are recovered but his body is missing, and Magath pieces together that Zeke had faked his death and escaped with the forces from Paradis. He calls an emergency meeting and summons the remaining Warriors; detailing his recent discovery, he believes that Zeke has been working with the enemy for at least four years. He announces that a global military alliance is forming and will conduct a scorched-earth operation to utterly destroy Paradis Island in roughly six months. However, Reiner insists on launching a surprise attack immediately. 
Magath leads Marley's retaliation attack against Paradis, coordinating the attack from one of Marley's airships. After receiving a smoke signal from Pieck and Galliard to let him know that they have engaged Eren, Magath orders his forces to launch their attack. He takes a group of soldiers and parachutes into Shiganshina, where he meets up with Pieck and Gabi. After chastising Gabi for putting herself in danger, Magath discusses with Pieck the likelihood of Eren using the Founder. Gabi's revelation that Paradis now has a Titan of royal blood according to Zeke leads Magath and Pieck to conclude that Zeke himself is probably this Titan and that they must prevent Eren from coming into contact with him and activate the Founder. Manning the anti-Titan artillery harnessed by Pieck, Magath declares that they are going to have Eren devoured and take back the Founder.
Pieck takes Magath to the top of Wall Maria, where he will have a clear shot at Eren's Titan with the artillery. He manages to score a direct hit to Eren's head with his first shot, impairing Eren's motor functions. Magath fires a second round into his head to further cripple him, intent on forcing Eren to use up all of his strength regenerating from his injuries. However, as Reiner struggles to devour Eren, he is knocked away by debris thrown by Zeke, with Magath displeased to see the arrival of the "Boy Wonder."
- Zeke Yeager - Magath was overseeing the Warrior program when Zeke joined and initially saw him as worthless and not worthy of becoming a Warrior. He would often belittle Zeke's efforts but eventually Magath became impressed with Zeke's consistent efforts and began to view him in a more positive light after Zeke exposed his parents as the leaders of the Eldian Restorationists; Magath would ultimately select him to become a Warrior and the next inheritor of the Beast Titan. While he expects Zeke to display the respect towards Marleyan authority that would be expected of any Eldian, he nonetheless openly expresses how impressed Zeke leaves him with his words and actions. He is willing to disclose valuable military information with Zeke, and even accepts his advice on the issues faced by such matters. The relationship between the two is severed after the Raid on Liberio after Magath realizes that Zeke was actually working against Marley.
- Willy Tybur - While only having known Willy Tybur personally for a month, Magath developed a strong trust with the nobleman. Very soon after their first meeting, Willy understood that Magath possessed more knowledge of the truth behind their nation than he let on, and the former was willing to discuss the nature of the Tybur family's authority and even the possession of the War Hammer Titan with him. Together, the two had conspired to bring about great changes for their nation, which Willy believed is in need of another "Helos."
- Colt Grice - Magath looks down on Colt for being an Eldian, as he does with all others of that nationality. However, he shows some value for him as a soldier, being concerned for his well-being when he rescued his brother Falco from enemy fire and listening to his ideas when he pitched them, even if he was going to deny them.
- There is an inconsistency in Chapter 91 in which Magath appears in several panels wearing the Eldian collar badge instead of the standard black clip.