Frieda Reiss

Frieda Reiss (フリーダ・レイス) was Rod Reiss' eldest daughter and Historia's half-sister. She is known to appear in Eren and Historia's dreams.

Human Form
When Eren dreams of her, he first mistakes Frieda for Historia, highlighting the sisters' physical similarity. She had long, smooth dark hair, stopping well past her shoulders. She appeared to be well dressed, given that she was a member of the royal family.

Titan Form
Upon transforming, Frieda's Titan form has pure white hair with dark loops within her eyes. Her torso has a defined muscle lining, much like the Female Titan's. Like other Titan Shifters she is physically fitter than normal, mindless Titans. Her Titan form was about 15 meters tall.

Personality
Frieda tended to be a good and caring person. She liked to walk and look after the peasants. She loved Historia to the point that she deleted her memories in order to protect her. She was brave and believed that her ancestors could not make her change.

After receiving the Coordinate, her personality changed. She suffered from depression and sometimes her behavior changed from her regular self to another much more aggressive. She also thought that humans were all sinners who deserved a punishment. However she fought to the death against Grisha Yeager in order to protect her family.

The Uprising arc
She appears in Eren's dream, with him seeing her from her own perspective while she looks into a mirror. However, in Historia's dreams she actually interacts with Historia. She teaches her how to read and tells her to grow up to be "ladylike" and a kind person. Frieda appears moved when Historia claims that she wants to be like her, then proceeds to manipulate Historia's memory. Telling her to forget about her until they see each other again.

While Hange and the new Levi squad attempt to search for Eren and Historia, Hange reads a report commissioned by Erwin on the Reiss family grounds. Rod's eldest daughter Frieda was loved by all in their village for her genuine character. She was even known to visit the fields to thank the farmers for their work. A tragedy struck the Reiss family the night Wall Maria fell. Bandits supposedly raided a chapel in the village, and killed the entire Reiss family save for Rod and Historia.

However, it is revealed through Eren's inherited memories -- triggered by Rod and Historia placing their hands on his back while he is chained up in the cavern -- that the murderer was actually Eren's father Grisha. In his Titan form, Grisha invaded an underground cavern under the Reiss family chapel, where the family had gathered to pray. He battled Frieda and consumed her, thus gaining her powers and memories. He then slaughtered her mother and siblings, and finally destroyed the chapel. When Eren later eats Grisha, Frieda's memories are transferred to him along with Grisha's.

Rod later explains that Grisha entered the chapel seeking Frieda's Titan power, and they fought one another in their Titan forms. Rod says Frieda possessed an invincible power but didn't have the experience to use it to its full potential. Therefore she lost to Grisha, who ate her and thus stole her power.

According to Rod, when Frieda was 15, she injected herself with the Titan serum and transformed for the first time in the same cavern below the chapel. Her uncle Uri Reiss, was chained up so she could devour his upper body. She received not only the Titan powers that Uri had inherited from his father, Frieda's grandfather, but the memories of the world before the Walls. She essentially became the "living book" of humanity and had the power to spread the information if she chose. But like all the others before her, she chose not to.

Rod tells Historia that Frieda isn't really dead, as her memories are still alive, and he asks her if she wants to see her sister again. Before she is about to inject herself and eat Eren, Historia flashes back to her childhood again. Frieda plucked a splinter out of her palm that Historia had gotten while trying to climb over a fence, then screamed at her for trying to do so. Historia recalls that Frieda would sometimes seem possessed, ranting about "sinners", then become deeply depressed afterwards. She wondered if Frieda was troubled by the burden of the world's memories. Rod tells Historia that yes, Frieda had also inherited the ideology of the first Reiss King, which is why she had never chosen to exterminate the Titans. As he continues to press Historia to inject herself, she remembers Ymir telling her to live for herself. And she gains the strength to throw Rod to the ground and rush to Eren's rescue.

After Historia kills his father, she receives some memories from her father. In one of them, Frieda was telling him that she will not give in to their ancestors's ghosts.

Abilities

 * Memory Manipulation: She is shown to be able to manipulate the memories of a person, she uses it to have Historia forget about her.This is related to the Invincible Power.

Titan Shifting
Other than the regular abilities possessed by Titans and Titan Shifters, nothing especially notable was seen belonging to her Titan form as she was killed rather easily by Grisha's Titan form.
 * Regeneration: Like all Titans, she possessed healing abilities. When punched in the face by Grisha, she started emitting steam from her wounds.
 * Enhanced Strength: She was briefly capable of withstanding Grisha's attacks.
 * "Invincible Power": Briefly mentioned by Rod. He said her Titan form held an invincible power, but she wasn't experienced enough to control it. Rod also goes on to mention it had the potential to exterminate all the Titans.

Trivia

 * In mythology, the name Frieda is literally derived from the Goddess Freyja, a central character in many Nordic myths. Her name is likely one of many references of real life Norse mythology alongside such "Ymir" and "Utgard".
 * It should be noted that Freyja is part of a saga involving a war between Vanir and Aesir, where the settlement of the latter is attacked by having their walls destroyed, bearing several similarities to the world of Attack on Titan. The Aesir are the principle deities of the Nordic mythology, such as Tyr, Thor, & Odin. Whereas the Vanir are a group of gods associated with fertility, wisdom, nature, magic, and the ability to see the future. With Freyja being one of the principle Vanir in the conflict, and is traded in a hostage negotiation to become one of the Aesir. They collectively oppose the Frost Giants for the remainder of the Edda's.
 * Freyja is also a Goddess involving Shamanism, which allows transformation by having one's soul seethe out the neck.
 * Frieda is Old High German, (fridu), which stands for "peace".
 * Her name also has an Old English meaning, (þryð), which stands for "force" and "strength".
 * She used to bite her hand in order to trigger the transformation. That explains why Eren used this method without even thinking of it.