|The Reiss family...is the true royal bloodline.|
|— Djel Sannes admits the truth|
The Reiss family (レイス家 Reisu-ke?) is the current royal family of Eldia that descended from the Karl Fritz, the First King of the Walls. They branched off from the original royal family after the Great Titan War, masqueraded as a minor noble family living inside northern Wall Sheena, and set up a false King and royal family to further draw attention away from themselves. They passed the Founding Titan between ruling monarchs every 13 years, until it got stolen by Grisha Yeager in the year 845. Currently, Historia Reiss, the illegitimate daughter of the late Rod Reiss, is the queen and the last known living member of the family.
In ancient times the royal family was known as the Fritz family. The family was descended from Ymir Fritz, the first of all Titans, whose descendants founded the nation of Eldia. However, towards the end of the Great Titan War, Eldia lost most of its territory to Marley, save for Paradis Island. King Karl Fritz moved Eldia's capital to Paradis Island and, with power of the Founding Titan, constructed the Walls using countless Colossus Titans. After he gathered as many Eldians as he could on the island, he wiped their memories of the outside world, making the people believe they were the last of humanity in a world overrun by the Titans.
However, this brainwashing did not affect those of minority bloodlines that were not Subjects of Ymir, such as the Ackerman clan and the Asian clan. Those who agreed to the wishes of the First King of the Walls were given the status of nobility. The Ackerman clan and the Asian clan, however, disagreed, and were hunted to near extinction for this. To protect his subjects, the First King sent an ultimatum to the Marleyan government, stating that if they were to interfere with his vision of peace, the millions of slumbering Colossus Titans would be unleashed upon the Earth. However, this ultimatum was merely a bluff to ensure a lasting peace. In secret, Karl Fritz made a vow renouncing war with the Founding Titan, ensuring that he and his successors would never use its power to wage war and would simply stand idle should the world invade in retribution. After this, the royal family that moved to the island cast aside the Fritz name and renamed themselves the Reiss family. Some members of the royal family decided to stay on the continent now controlled by Marley, however.
Some time later, King Reiss gave the Founding Titan to a successor, dying in the process. The Reiss family ruled the Walls in secret for the next 100 years by masquerading as a minor noble family living inside of Wall Sheena near the Orvud District. The King of the Walls would be their figurehead.
A special ritual was conducted every generation in which the Founding Titan was passed down from one family member to another. The current inheritor of the Founding Titan would be chained in the cavern beneath the village chapel near the Reiss family estate. Meanwhile, the successor to the Founding Titan, would inject themselves with a serum and transform into a Pure Titan to devour their predecessor. This would give them the power of the Founding Titan and the lost memories of humanity inside the Walls. However, this would also give them the ideology of the First King and force them to keep humanity inside the Walls under the Titans' rule, even if they previously had a desire to free them.
They would never reveal the reason for this, even to their own friends and families. While inheritors of the Founding Titan retained much of their original personality, whenever using the Founding Titan's power or falling under the will of the First King, it would become visible in the user's eyes by a darkening of their irises. This was apparent in Uri, Frieda, and Rod and Uri's father. The Reiss who currently possessed the Founding Titan would be the true king or queen of the Walls for however long they possessed it, as shown by Uri and Frieda.
One of the many Founding Titans was Rod and Uri's father. The brothers begged him to exterminate the Titans, but he always refused, so much so that he locked Rod behind bars for his attempts. When it came time to pass his position to a son, Uri volunteered and asked Rod to pray. After the procedure, Uri had inherited the First King's ideology, and wished to build a paradise for mankind inside the Walls. One of the ways he did this was by stopping the persecution of the Ackerman clan after a failed assassination attempt by Kenny Ackerman, who later became a Reiss bodyguard and Uri's closest friend.
Years later, Rod married and had five children with his wife; Urklyn, Frieda, Dirk, Florian, and Reiss. Rod also conceived a sixth child, Historia, with Alma, his mistress, and confidant; one of the Reiss family maids.
Alma gave birth to Historia on her parents' farm, which was on land controlled by the Reiss family. Historia had an isolated upbringing and was raised to believe she was an unfortunate and unwelcome mistake. She had no friends and could never leave the farm. Her mother was distant to her, only speaking to her twice, and both times wished she did not exist. Her grandparents rarely spoke with her either. This led Historia to developing a martyr complex. She wished to die in a way in which everybody would remember her fondly and not hate her for existing.
However, Historia was not as alone as she believed. Her half-sister Frieda would visit her in secret and taught her to read. Frieda encouraged her to be a lady, like the girl in Historia's book; a kind girl who was always thinking of others. After her visits, Frieda would wipe Historia's memory to protect her. For a while, Historia would only recall those memories in dreams and forgot them when she awoke.
When Frieda was fifteen, she inherited the Founding Titan by devouring Uri in the Reiss family's succession ritual. She was well loved on her family's estate and would be known to go out to the fields to thank the farmers for their work. However, after becoming the true queen inside the Walls, she began to experience bouts of mania followed by depression, such when she discovered that Historia had tried to leave her family's farm by crossing the fence.
During the fall of Wall Maria in 845, the Reiss family goes to the village chapel to pray. However, Grisha Yeager, a man with the power of the Attack Titan meets them there. He begs Frieda Reiss to save the people inside the Walls from the Titans, but she refuses.
Grisha attacks the Reiss family and transforms into the Attack Titan. He defeats Frieda, who is not experienced enough to fully control the power of the Founding Titan, and devours her, stealing her Titan. He then proceeds to exterminate the rest of the family. Grisha kills Rod Reiss's wife and children and destroys the chapel. Only Rod survives the massacre and Historia Reiss is unharmed by virtue of having not been present.
With Frieda's death, Rod succeeds her as the true king. To cover up Grisha's attack, he blames the chapel's destruction and his family's death on bandits.
Since the rest of his children are dead, Rod goes to the farm and to retrieve Historia since she is of the appropriate bloodline. However, the First Interior Squad interferes, having been sent by the Assembly to kill Historia and her mother, Alma. This is because they view Historia, an illegitimate child, as unfit to bear the Reiss name.
Rod decides on a way to keep Historia alive. She would give up her old identity, forsake any claims, and join the Survey Corps, hopefully dying to a Titan. This is seen as an acceptable alternative, and Historia is given the identity of Krista Lenz and sent to live in a refugee camp until she is old enough to enlist in the military.
Once enlisted, Rod sends the Church of the Walls to monitor her. Under her Krista persona, Historia is the lady Frieda wanted her to be, but also constantly seeks to die in a way that would be considered a martyr. She graduates 10th in the 104th Cadet Corps because Ymir, her best friend, slacks off the last several weeks of training to give Historia the 10th spot so she can join the Military Police. Historia joins the Survey Corps with Ymir, in a squad under the command of Nanaba, due to her martyr complex.
Since Historia is a descendant of Karl Fritz, she is potentially valuable to the Marleyan government. Ymir realizes this and forces Reiner Braun and Bertolt Hoover to work with her so she can be pardoned for eating their friend and mission partner, Marcel Galliard, when she was trapped in her Titan form. She succeeds in capturing Historia, but Reiner and Bertolt are defeated by the combined efforts of the Survey Corps and Eren's newly awakened power to control nearby Titans. Ymir decides to leave Historia behind for her own safety and a devastated Historia is pulled away by the Survey Corps.
Shortly after, Levi and Hange Zoë interrogate Djel Sannes, a member of the Interior Military Police, who reveals that the Reiss family is the true royal family, meaning that Historia has a chance to claim the throne. Upon learning this, Erwin Smith plans to overthrow the current monarchy and install Historia as queen.
Historia initially refuses, but after being pressured by Levi, she agrees. However, in an attempt to track down her father Rod Reiss, Historia and Eren are captured by Kenny Ackerman and taken beneath the village chapel to the chamber where the Reiss family passes on the Founding Titan as part of their succession ritual. Rod attempts to persuade Historia to transform and devour Eren, saying that she would be able to meet her sister Frieda again and become strong enough to kill all the Titans.
Historia considers reclaiming the Founding Titan, but learns that all the previous rulers of Reiss family fell under the sway of the First King's ideology, her sister included. She would not be able to change a thing. Her father is only using her to regain the Reiss family's power. Historia remembers Ymir's words to live her own life and smashes the Titan injection. She attacks Rod and takes his bag with the keys so she can free Eren.
Sobbing, Rod crawls on the floor to the evaporating Titan serum while calling the names of his dead family members. He licks the serum which transforms him into the largest Titan in existence, a 120-meter Abnormal Titan. His transformation is so powerful it collapses the cavern around it. Now a Pure Titan, he begins to crawl towards Orvud District.When the Garrison and the Survey Corps plan a defense against Rod's Titan, Historia informs Commander Erwin Smith that she will be participating in the battle. Regardless of who kills the Titan, she wants the Survey Corps to credit her with the kill to unite the people behind her as their new queen.
When the Survey Corps uses explosives to blow up the head and torso of the gargantuan Titan, Historia is among the soldiers who attack the various chunks of flesh in an attempt to find the nape. Historia herself delivers the final blow, and sees visions of her father's past when she kills him. She lands on a cart in the middle of Orvud District, where she declares herself the true ruler of the Walls.
Historia is crowned queen, but in practice the military becomes the true power of the state. Though a figurehead, Historia is still admired by her people for having vanquished a Titan and being a caring individual. She uses her family assets to start a farm to care for orphans and other people in need.
No longer part of the Survey Corps, Historia remains behind during the operation to reclaim Shiganshina. When the soldiers return with the truth of the outside world, she sits in on the military meeting and sides with Dot Pixis in releasing that information to the public. She later bestows medals of honor to the nine surviving soldiers of the Survey Corps.
When the Anti-Marleyan Volunteers reach Paradis Island, Historia attends the military meeting regarding whether to cooperate with them. Eren reveals that he can use the Founding Titan's power if he is in physical contact with a Titan of royal blood, and this will allow them to get around the First King's vow to renounce war. Historia appears surprised when Eren says he refrained from speaking of this sooner because he was concerned that she might be turned into a Titan so he could use it.
When the first Hizuru ambassadors arrive on Paradis, Historia listens to Kiyomi Azumabito go over the processes that must be in place to secure the island's safety. One of them is for Zeke to pass down his Beast Titan to one with royal blood. The next is for the one with royal blood to use the remainder of their 13 years to bear as many children as possible. Although the suggestion causes much controversy among those present, Historia agrees to the plan. By 854, she is pregnant and now bears the burden of constantly passing down the power of the Titans through her descendants just like her predecessors. 
- The Reiss family is one of four notable bloodlines mentioned in the manga, the other three being the Ackerman clan, the Asian clan, and the Tybur family. The Fritz family is also of great importance, though their bloodline is identical to the Reiss family.
- In the Q&A section of the August 2018 edition of Bessatsu Shōnen Magazine, Hajime Isayama answered a question regarding a hypothetical mixing of the Ackerman and royal bloodlines:
Q: If a child were to be born between two people from the royal family and the Ackerman clan, which blood would take precedence? ("Chewing Gum" from Chiba Prefecture)
A: I think that both [of their bloodlines] would be reflected [in the child] as usual. (Isayama-sensei)
- Presumably this means that the child would have the "awakened power" of the Ackermans and can become a Titan with royal blood—able to activate the Founding Titan and fully utilize the power of the Titans—like the members of the Fritz/Reiss royal family.