Also known as the first Children. She was the first chosen pilot for project EVA.
She is perhaps the most interesting character in the series. She is an artificial life created in a laboratory. Because of that, she does not have the emotions that normal human beings take for granted. In contrast to the other characters who tried to bury their emotions, she gradually developed her emotional self throughout the series.
In the beginning of the series, she expressed little emotion and followed orders explicitly like a robot. She spoke in a very terse and objective manner, bearing little emotion: "An emergency call ...... I'll go first ..." she said to Shinji in the third episode when Tokyo-3 was alerted to the presence of the 4th Angel. Gendou showed concern and affection towards her by rescuing her from the plug in episode 5, but she did not quite know what to make of it, yet she felt something (She kept the cracked glasses of Gendou in her apartment). There was a similar situation after the 5th Angel was neutralized in episode 6 when Rei almost got killed: "Why are you crying? I'm sorry. I'm sorry. I don't know what to feel at times like this..." she said. Shinji suggested that she should smile and she did. There, she first showed a glimpse of emotion towards another. In episode 23, after self examination, she realized that perhaps she felt sad about being alone. She was able to express emotion for the first time - crying.
Asuka called her a robot without autonomy. Because her life was artificially created, she constantly questioned her own autonomy. Rei is an artificial soul placed inside a shell. But Rei realized that her existence is real. During the mutual compatibility test in episode 14, in EVA-01, Rei felt the presence of another entity other than herself. Similarily, when the 16th Angel penetrated her mind in episode 23, a separate entity in her own form appeared before her. Through distinguishing herself from another being, she was able to identify her own autonomy. Her history and interaction with other people makes her real and shaped her into who she is today. Her feelings towards Gendou and Shinji shaped her into who she is today. What was her is Rei. Like everyone else, she grows and matures into the person she is.
Rei is not the only one who has to realize her own autonomy. In the final episode, Shinji also learned this. Without other people, he cannot know his own form or himself. Only through the existence of other people and comparing the differences can he come to know himself.
A mystery that lingered is Rei's true identity. In episode 24, after Nagisa descended to the Central Dogma, another AT field was detected for a brief moment and disappeared. That was followed by a brief screenshot of Rei standing on the fringe of the Central Dogma. That, together with Nagisa stating that he and Rei were the same seemed to suggest that Rei could in fact be an Angel. In addition, the second Angel was never mentioned in the series.