The Reiss family (レイス家 Reisu-ke?) is the true royal family of humanity, disguised as a noble family living in the interior. They are descended from the first king, who built the Walls using the Founding Titan's power. The Reiss family then passed the Founding Titan down for many generations in the Underground Chapel.
Following the fall of Wall Maria, Grisha Jaeger attacked and slaughtered most of the family, taking the Founding Titan with him. After Rod Reiss's death, the only living member of the family is his illegitimate daughter, Historia, who is the current Queen of the Walls.
A century ago, the First Reiss King used the Founding Titan's power build the Walls and erased the memories of humanity inside. Some people were unaffected by this. Two bloodlines who were immune, the Ackermann family and the Oriental clan turned their backs on the king. Fearing his inability to control them, the king ordered their prosecution and both bloodlines were hunted to near extinction. The king also built the Underground Chapel for his family to pass down the Founding Titan.
Sometime before the fall of Wall Maria, Historia Reiss was illegitimately born through an affair between the head of the Reiss family and a mistress. It was debated whether or not Historia should be killed to alleviate the issue of illegitimate inheritance, but by the time Wall Maria fell it was decided that the child could live a humble life under the alias "Christa Lenz." This issue was discussed among the Order of the Walls and Ymir overheard the conversation while stealing in the interior.
A few years later, Ymir joined the 104th Cadet Corps after hearing that Christa had joined as well and eventually she found Christa among the cadets. One day during snowstorm training Ymir revealed to Historia that she was aware of her true identity and backstory, realizing that her nearly suicidal choices were out of a desire to die to escape her past. Ymir then used the power of her Titan to save a fellow cadet from death by descending a cliff when Historia was unaware. She later asked Ymir about how the feat was possible and Ymir chose to reveal her powers to Historia under the condition that when her powers became known, she would go back to living under her true name.
On the day of Wall Maria's fall, the entire Reiss family, save for Rod Reiss is slaughtered by Grisha Jaeger. He transforms into a Titan and devours Frieda's Titan, obtaining the Founding Titan from her.
After losing his entire legitimate family, Rod seeks his illegitimate daughter Historia, who is living on a farm owned by the Reiss family. He tells Historia she is coming to live with him, but Alma accidentally admits Historia is hers, and thus an illegitimate child born from an affair. Kenny Ackermann reminds Rod of the consequences of this, leading Rod to abandon his daughter. To spare her life, Rod sends Historia away with a new name—Christa Lenz—to live a quiet life.
Years later, during the Wall Rose invasion, Ymir's Titan powers are used to save her fellow Scout Regiment recruits from Titans at Utgard Castle. Staying true to her promise, Historia reveals her true name to Ymir and the others. Pastor Nick reveals to Hange Zoë that Historia is technically allowed to disclose details about the nature of the Walls that regular members in the know are barred from revealing. Later, after Historia speaks with Hange Zoë, Hange briefly questions Historia about her family and their nobility, which Historia confirms to be her true lineage.
Historia and Eren are captured by Kenny Ackermann and taken to meet Historia's father, Rod Reiss. Having lost track of the two abductees, Levi and Hange torture Djel Sannes for information regarding the Reiss family. Sannes eventually reveals that the Reiss family is the true royal family, with the currently recognized King being a puppet monarch. Upon learning this, Erwin Smith plans to overthrow the current monarchy and install Historia as queen.
After relocating to the Underground Chapel on the Reiss estate, 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 is told how all the previous rulers of Reiss family fell under the sway of the First Reiss King's ideology, her sister included. She would not be able to change a thing. Kenny argues how her father is only using her to regain the Titan power. After considering the situation, Historia rejects the plan by smashing the Titan injection, throwing Rod to the floor, and going to free Eren.
Sobbing, Rod licks the evaporating Titan serum while calling the names of his dead family members. This transforms him into the largest Titan in existence, a 120-meter Abnormal Titan. Now a Pure Titan, he begins to crawl towards Orvud District.
Before battle preparations, Levi bluntly informs Historia of the plan to install her as queen after the battle. Historia agrees on the condition that she will be participating in the battle. Regardless of who kills the Titan, she wants the Scout Regiment to credit her with the kill to unite the people behind her as their new queen.
When the Scout Regiment blow up the body of the enormous 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. Landing in the streets below, she declares to a surrounding crowd that she is 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. She uses her family assets to start a farm to care for orphans and other people in need.