|Reiss is the true royal family.|
|— Djel Sannes states the true royal family|
The Reiss family (レイス家 Reisu-ke?) is the current royal family of Eldia that descended from 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 Sina, 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 was stolen by Grisha Jaeger in the year 845. Currently, Historia Reiss, the illegitimate daughter of the late Rod Reiss, is the queen and 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. The 145th Fritz King moved Eldia's capital to Paradis Island and, with power of the Founding Titan, constructed the Walls using countless colossal-sized 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 Ackermann clan and the Oriental clan. Those who agreed to the wishes of the First King of the Walls were given the status of nobility. The Ackermann family and the Oriental 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 Titans would be unleashed upon the Earth. However, this ultimatum was merely a bluff to ensure a lasting peace. In secret, the First King 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 Sina near the Orvud District. The King of the Walls would be their figurehead.
On the day of Wall Maria's fall in 845, the Reiss family goes to the village chapel to pray. However, Grisha Jaeger, a man with the power of the Attack Titan invades the chapel and meets them there. He begs Frieda Reiss to save the people inside the Walls from the Titans, but she refuses. Grisha decides to transform into a Titan to steal the Founding Titan from Frieda. Frieda transforms into a Titan and battles Grisha, but is defeated due to not being experienced enough to fully control the power of the Founding Titan. Grisha devours Frieda and obtains her Titan, and then proceeds to slaughter the rest of the Reiss family, killing Rod Reiss's wife and children and destroying 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.
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.
Under the identity of Christa Lenz, Historia enlists in the military years later. Once enlisted, Rod sends the Order of the Walls to monitor her. Under her Christa 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 Scout Regiment with Ymir, in a squad under the command of Nanaba, due to her martyr complex.
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 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.
No longer part of the Scouts, Historia remains behind during the operation to reclaim Shiganshina. When the soldiers return with the truth of the outside world, she presides over the military meeting and sides with Dot Pyxis in releasing that information to the public. She later bestows medals of honor to the nine surviving soldiers of the Scout Regiment.