The Reiss family (レイス家 Reisu-ke?) is a noble family living in the interior.
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.
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. Later, after Historia speaks with Section Commander Hange Zoë for Ymir's sake, Hange briefly questions Historia about her family and their nobility, which Historia confirms to be her true lineage.