|Every country out there is frothing at the mouth, just waiting to get Marley when it's weak. I know, because my family came here from an internment zone in another country. It was awful...The hostility that Eldians face here is nothing compared to the way it is in other countries.|
|— Udo explains global hostility against his people|
Udo (ウド Udo?) was an Eldian Warrior candidate (戦士候補生 Senshi Kōho-sei?, also translated as "Warrior Cadet") working for the Marley government. Before his death, he was a candidate to inherit the Armored Titan power.
Udo was a boy of average height and build, though easily the tallest and heaviest of the Warrior candidates, with shaggy black hair and dark eyes which he wore oval-shaped goggles over in place of glasses. Like his fellow Warrior candidates, he wore the standard Marleyan soldier uniform consisting of a light jacket and pants, supply packs on the hip, suspender straps, a hard hat with a stripe around it, and tall combat boots. As an Eldian his uniform was different from that of the Marleyans in that the shirt collar has a small badge with the Eldian star instead of a small clip and the hat has a white stripe with the Eldian star on it instead of a pure black stripe. As is law in Marley, Udo also wore the Eldian armband as all his family and ancestors residing there do as well.
Udo was not the most secure soldier nor did he possess a very level head. Like his other Warrior candidates, he was trained to aid the wounded and cared enough to come to Falco's aid, but his shaky nature sometimes causes him to neglect those he is treating. While he was not an excelling soldier, he possessed some margin of skill to have survived for four years in the Marley Mid-East War.
Udo and his family used to live in a foreign country before being moved to Marley, undergoing hatred worse than Marleyan hostility.
During the Marleyan war with the Mid-East Allied Forces, Udo is present among 800+ other Eldians fighting on behalf of Marley. However, he and some of his friends such as Zofia, Gabi, and Falco are valued as Warrior candidates with the potential to inherit the Armored Titan power. When Falco is brought back to the hiding trench by his brother Colt, Udo and the other Warrior candidates rush to his aid and begin treating his injuries and bandaging his wounds. During this time, Udo begins to panic in fear of their military failing and wraps Falco's bandages too tightly causing Gabi to scold him. Falco wonders aloud why they are on the front lines as candidates and Gabi claims it is so they can be evaluated for their qualifications for the Armored Titan power. Udo and Zofia then boast about their superior grades above Gabi, who in turn expresses her resolve to kill the people of Paradis Island. As Udo and Zofia continue to treat him, Falco asks Gabi about the item she is fiddling with and she grins as she shows it to all of them.
When Gabi offers to take out the enemies anti-Titan artillery solo, Udo watches her execute her plan of attack and once she is successful in derailing the train, she begins to flee only to be shot at by one final surviving gunmen. This pushes Falco over the edge and he jumps out of the trench to try to save her as Udo and the other Warriors watch in fear. However, an ally named Galliard transforms into a Titan and shields Gabi from gunfire while she and Falco collide as they both dive for the ditch. Udo and the others then watch as Galliard races off and kills the last gunmen.
When Falco returns to the trench with a wounded Mid-East soldier, Udo helps tend to the soldier's wounds. The soldier mutters something in a different language and Udo helps translating what he said.
After the battle, Udo is seen with the rest of the Warrior candidates in Fort Slava's harbor, watching as surviving Mid-East soldiers are sent back to their home countries. Udo grows angry, believing that the soldiers' war stories will only serve to demonize Eldians to the rest of the world, and begins lashing out at inanimate objects around him until Reiner arrives. He is later seen at a party being held in Gabi's honor during the journey back to Marley.
Upon arriving at Liberio, Udo is reunited with his family. When the Warriors and Warrior candidates are summoned to their headquarters soon after returning, Udo complains at how little time they had to rest after the Fort Slava battle.
During training, Udo congratulates Falco who beat Gabi for the first time. On the way back to the internment zone, he enthusiastically shares the news with the soldiers and comments on how exceptional it is. Later that day, he watches the preparations for the festival with Zofia and Gabi. Udo tells about his knowledge of the world's hatred towards their people.
At night, the candidates serve the ambassadors who came to Liberio. Hearing guests insulting Eldians, Udo gets distracted and spills some wine on an ambassador from Hizuru. He apologizes but the woman tries to calm him down. Udo insists, and they are interrupted by a man who leads the ambassador away. As he asks her about her motivations, he is shocked to learn she helped him despite knowing his origins.
The following day, Udo enjoys the festival with his friends then joins the Warriors to listen to Willy Tybur's speech. As the stage play starts, Udo and Gabi wonder why Falco and Reiner have not returned yet. When Willy reveals the truth about the Great Titan War during the play, Udo and his fellow Warrior candidates are stunned to learn that the 145th King, Karl Fritz was the one who put an end to the war.
When Eren Yeager suddenly transforms into the Attack Titan behind the stage, detonating the house he was currently hiding in, debris from the broken house is hurled into the audience. A flying piece of debris lands on and crushes Zofia, killing her. This causes a shocked Udo to attempt to free her from the rock, but before he can do anything for the already dead Zofia, he is trampled by the panicked audience members trying to escape Eren's Titan.
As the crowd flees, his head is split against the rubble and his neck is snapped by the trampling. Colt and Gabi recover his body, though Colt is unable to say whether or not Udo is dead. They rush Udo to the hospital amidst the chaos. At the hospital, the doctor refuses to give Udo a bed, saying there is no space and he is already dead. After grieving, Colt lays his corpse down on a sheet outside.
- Falco Grice - Falco and Udo are comrades as Warrior candidates for the Armored Titan power. As such, they have grown closer in the four years they spent at war with the Mid-East Allied Forces and care for one another. When Falco returned from battle wounded and delirious, Udo treated his wounds to the best of his ability, despite wrapping his bandage too tight in the midst of a personal panic.
- Gabi Braun - Gabi and Udo work closely together on the battlefield and show a liking towards one another. Gabi is kind to him as she is to all the other recruits, but they appeared to get in a small spat when Gabi yelled at him for panicking and wrapping Falco's bandage too tightly. However, this does show that the two are close enough to exchange opinions without being angry.
- Zofia - As candidates, they worked well together in the midst of war, but showed very little interaction with one another. Zofia seemed to find humor in Udo's blatant panic and saw him in a friendly yet foolish light, but the two were not unkind to one another. During the chaos from Eren Yeager's attack on the Liberio festival, Udo futilely attempted to free Zofia's already-dead body from underneath a boulder, and he was ultimately trampled to death by the fleeing crowd while attempting to save her.
- Udo ranked thirty-sixth in the Third Character Popularity Poll.
- Sketches of Udo included in the Attack on Titan: Final Manga Exhibition revealed that Udo was planned to have the full name Udo Bock (ウド・ボック Udo Bokku?).
- "Bock" is a style of beer that originated in Germany.