Hopefully this image can help as reference for costume making.
Personality
"Whatever else, she needs to be painted in as a bitterly unhappy young girl with little sense of presence." Hideaki Anno, to Yoshiyuki Sadamoto on Rei's character design
At the start of the series, Rei is socially withdrawn, seemingly emotionless, and rarely interacts with anyone, except for Gendo, to whom she initially displays loyalty but with whom she has a generally distant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 says in episode 15 that she "doesn't know what kind of person he is"). She lives by herself in a drab, dingy, sparsely-furnished apartment in Tokyo-3, a clutter of books being her only apparent luxury items (assuming they are not, in fact, schoolbooks). Toward the end of the series, Shinji comments that her apartment is very similar to the place where Ritsuko says Rei was born. Rei's neutral personality and emotionless nature arouse the scorn of Asuka Langley Soryu, who at one point refers to her as a "doll" or "puppet." Rei is, in many ways, the exact opposite of Asuka. While Asuka is extroverted, Rei is introverted. Also, Asuka is extremely boastful and overly proud of her accomplishments, while Rei constantly states her life has little value. Their appearances are also, in ways, inverses of each other (Rei has short blue hair and red eyes, while Asuka has long reddish-blond hair and blue eyes).
"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.
Rei implies in episode 12 that she is a vegetarian, as she states that she "dislikes eating meat". The reason for this is not explained, though both Anno and Nadia (the title character of Anno's earlier anime) are vegetarians as well.
It is implied in episode 19 that Rei does not dream. When Shinji is unconscious in a hospital Asuka says he must be dreaming, and Rei is confused about what Asuka is referring to.