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's and Historia's dreams.

Human Form
When he dreams of her, Eren first mistakes her for Historia, which shows that they must look similar. She has long smooth dark-coloured hair.

Titan Form
In Titan form, Frieda's appearance changes quite drastically. She develops a huge increase in height, pure white hair, and like other Titan Shifters, she is more physically fit compared to normal Titans. Upon transformation, her eyes convey dark loops and her torso a defined muscle lining, much like the Female Titan's.

The Uprising arc
She appears in Eren's dream, with him seeing her from her own perspective, while she looks into a mirror. In Historia's dream however, she actually interacts with Frieda. She teaches Historia how to read and tells her to become a kind person. Frieda appears moved when Historia claims that she wants to be like her and 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's location, Hange reads a report commissioned by Erwin on the Reiss family grounds. Hange explains that 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, and when bandits supposedly raided a chapel in the village, the entire Reiss family save for Rod and Historia was murdered.

However, it is revealed through Eren's memories that the one responsible was actually Eren's father, Grisha. Grisha in his Titan form slaughtered and ate Frieda and her family while they were hidden in a basement under the chapel, getting their memories. When Eren ate Grisha, Frieda's memories were transferred to Eren along with Grisha's.

Rod later explains that Grisha entered the chapel where the entire Reiss family was located, seeking Frieda's Titan power, and they both fought in their Titan forms. Rod says Frieda contained invincible power but couldn't control it up to its full potential, and lost against Grisha and he ate her, stealing her power. Rod tells Historia that Frieda isn't dead, as her memories are still alive and he asks if she wants to see her sister again.

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. Like many other Titans, she has regenaration and enhanced strength.

The Coordinate
Briefly mentioned by Rod. He said her Titan form held an "invincible power", but she wasn't experienced enough to control it. After eaten by Eren's father, her powers passed to Grisha Yaeger. However, when Eren ate his father, he gained to power. By this ability, Frieda is shown to be able to manipulate the memories of a person, when she had Historia forget about her. By touching Eren - who have these powers now - Historia gained her memory back. It is also triggered Eren's memory to remember Grisha's memories after gaining the Coordinate.

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 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.
 * Freyja is also a Goddess involving Shamanism, which allows transformation by having one's soul seeth out the neck.