Background

At the beginning of the series, little is known about Rei and her stoic personality puzzles her peers. As the series progresses, her personality evolves and she becomes more involved with the people around her, particularly her classmate and fellow Eva pilot Shinji Ikari. In the concluding events of the storyline, the audience learns that Rei's mysterious origins tie into the story's resolution. Her role in this conclusion is not made clear in the TV series, but is one of the principal plot points of The End of Evangelion.

No specific information is given about Rei's origin or heritage. Ritsuko Akagi states that Rei was born in a certain room deep in the lower levels of NERV headquarters, but this is all that is explicitly revealed in the series about her creation.[1] The Red Cross Book stated that Rei was created from the "salvaged remains" of Yui Ikari after Yui's absorption into Unit-01 in 2004. The connection between Rei and Yui is implied a few times during the series. Gendo introduces Rei to the NERV staff in 2010 as an "acquaintance's child" whom he is temporarily taking care of. In episode 21, Naoko Akagi says that Rei physically resembles Yui. The character model used in the 2010 scenes is based on development materials in which her age is only 4.

Kaworu Nagisa cryptically mentions in their only conversation that she is "the same" as him.This statement was expanded in the director's cut of the episode, in which Kaworu goes on to say that he and Rei are human vessels for the souls of Adam and Lilith, respectively. Later in the episode, she generates an external A.T. Field which is as strong as Kaworu's. In a pseudo-hallucinatory scene in episode 25, Gendo tells Rei that the day of Third Impact is the day for which she was created. In The End of Evangelion, she plays an important role in the Third Impact, although not in the fashion that was intended by Gendo.

Rei is shown spending unknown lengths of time in an LCL-filled tube underneath a structure resembling a giant brain in the Dummy System room, the reasons for which are not revealed. The circular wall of this room is originally depicted as being covered with DNA sequences, but near the end of the series, it is revealed to be an LCL-filled tank in which dozens of soulless clones of Rei are held. The clones are said by Ritsuko to be the cores of the dummy plugs; she also refers to them as "spare parts" and later as "vessels" for Rei. The movie pamphlet and volume 11 of the manga state that she can be replaced entirely, with her soul being transferred to a new clone body every time she dies, although her memory becomes muddled with each transfer. Other supporting evidence in the anime includes her earlier line that "if I die, I can be replaced."While they are the same character, these different incarnations are commonly referred to as "Rei I," "Rei II," and "Rei III," after episodes and musical pieces with the same names, and are denoted as such in the storyboards and script as well. In episode 23, the controller that Ritsuko uses to destroy the clones lists numbered clones from "Rei 004" and upwards. Rei I was the very first incarnation and was murdered by Naoko Akagi in 2010 (as shown in Episode 21). Rei II is introduced in the first episode and appears in the most episodes of the incarnations. She dies when she sacrifices herself and Unit-00 to destroy Armisael in episode 23. Rei III is seen from the second half of episode 23 through to The End of Evangelion.

In the final episode of Neon Genesis Evangelion, the audience is introduced to a pseudo-hallucinatory alternate reality. Here Rei's personality is completely different, and her behavior is more "normal" (for example, she shows embarrassment and anger when she thinks Shinji looked up her skirt, in stark contrast to her emotionless reaction to the incident with Shinji in Episode 05). The Evangelion spin-off manga Neon Genesis Evangelion: Angelic Days is set in this alternate universe, and its version of Rei is a central character.

In The End of Evangelion, a shining figure of Rei is shown for a few frames looking down at Misato and Ritsuko moments before they die. These spectral images also appear over the corpses of the slain NERV personnel. During Third Impact, multiple Reis appear in the NERV control room, seemingly apparitions created by Lilith. She also briefly watches over Shinji and Asuka in the final scene of the movie.

Personality

At the start of the series, Rei is socially withdrawn, seemingly emotionless, and rarely interacts with anyone, except for Gendo, to whom she initially displays loyalty but with whom she has a generally distant relationship. She lives by herself in a dull sparsely-furnished apartment in Tokyo-3. Toward the end of the series, Shinji comments that her apartment is very similar to the place where Ritsuko says Rei was born. Rei is also shown to have an interest in psychology and genetics, including reading a book in German on the latter subject in Episode 09.

"An emotional change causes certain muscles in the face to tense, producing an "expression". Rei is expressionless but is it that she doesn't feel emotion, or that she is merely unable to express it?" Yoshiyuki Sadamoto on Rei [emphasis in original].As the series progresses, she begins to develop relationships with others and to show moments of genuine emotion, to even become sad and cry. Her English voice actor, Amanda Winn-Lee describes her saying, '"Rei is not totally devoid of personality, otherwise she would not be interesting." There is a small spark of humanity, but it is "clouded by this huge sense of negative self-worth and the realization that she is expendable." The joy of playing Rei is exploring that small spark.' And also said: "I got into a weird mode - I can't describe it. It's a good thing I'm in a little padded room when I'm doing it because that's where she belongs. She knows she's expendable, but the thing is, she's still human so you can't do her totally catatonic."

In the manga adaptation, Rei appears to respond more readily to human contact than her anime version. Shinji's interactions with her are implied as the main reason of her change; by the time of the battle with the Angel Armisael, it is strongly hinted that Rei is in love with Shinji. Shinji reciprocates this affection to some degree, but is unsure of what he feels for Rei and where their relationship is going. In volume 9, he acknowledges that the gap between them has been considerably reduced since they first met; still, he hopes that this gap can be reduced further. Her desire to be recognized as a person, rather than as a "doll," puts her in some awkward, even dangerous situations. At one point, Ritsuko tries to strangle her when she talks back to the doctor.

In the battle against Armisael, when the Angel communicates with Rei, it attempts to make Rei believe that her soul is evil; that she does want Shinji for herself and is angry at Asuka for drawing Shinji's attention away. As in the anime, she self-destructs Unit-00 to destroy the Angel. However, it's hinted that the "new" Rei still has some residual memories as she wonders to herself why she is alive again after returning to her quarters.