She is a kind person with respect for her comrades.
After the arrival of the Colossal Titan and breaching Wall Maria, Historia had began her training in the military. Upon graduation she had placed 10th, a rank under Sasha Braus (this was due to Ymir giving her the spot).
After the breaching of Wall Rose, Historia wasn't seen going into battle until attempting to help Armin overcome his dread of supposedly losing his friend. She helped Mikasa and the other trainees retake the supply room while the rogue titan kept the other Titans distracted. She didn't make an appearance again until when she had shown her Vertical maneuvering equipment as the Military Police was looking for the one that killed the two captured titans. 
Despite Historia's fears in facing the titans again after going through the same thing that Jean, Yuka Mochida and the others, she stayed put and became a member of the Scouting Legion. There, she helped save Reiner, Armin, and Jean by supplying a horse after they were stranded from an encounter with the Female Titan.
She was seen up on the trees baiting the titans and preventing them from entering the forest as the Scouting Legion attempted to capture the Female Titan. After this she wasn't seen again until the Ape Titan attacked with the other titans.
When Pastor Nick revealed that to the members of the Scouting Legion that Historia may know about the walls having titans inside them because of her family, and that she went under an assumed name after being caught in the middle of her relatives' dispute. It wasn't long until Eren and the others realized that she was often around Ymir.
When she remembers being with Ymir when they were helping Daz in her hometown. She was confronted for living under an assumed name after being caught in the middle of a serious dispute between her relatives. Before transforming into a titan to fight the other titans attacking them, Ymir made Historia promise her to go back into living under her real name.
After Historia and the rest of the trainees watch Ymir fighting against the group of Titans nearly falling off the tower when Ymir transformed, to which Reiner questioned her is she knew about it but tells she didn't know herself despite being so close to her and refuses to believe it at first. When the Trainees question who's side Ymir is on, Krista remembers some events of her and along with the other watching Ymir's attempt to fight the other Titans and came to the conclusion that she is fighting to protect everyone. She begins to cheer for Ymir telling her not to die and to break down the tower and celebrates when Ymir does so. She, along with the others, ride on her back and land on the ground safely, except for Ymir who is eaten by the Titans. As Historia tries to make her way to Ymir to fulfill her promise she is nearly grabbed by a Titan but is saved by Mikasa. As everyone watches in disbelief at Ymirs wounds and the truth, Historia reveals her true name. 
She reveals her true name as Historia Reiss to Hanji and pleads with her to go easy on Ymir. While she knew what her friend did was wrong in hiding information that could've been useful to the Scouting Legion, she understood why she did it to protect her comrades. Hanji tells Historia she understands Ymir being a titan like Eren and being an ally of humanity, but the rules are not simple like she thinks. She watches with horror as Reiner in his Armored Titan form grabs Eren and Bertolt snatches Ymir in his Colossal Titan form.
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