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Tatiana Guiterrez is a side protagonist of The Evil Within.
Tatiana is a nurse in the locked ward of Beacon Mental Hospital. Though she is alluded to be supernatural, her primary purpose is to help Sebastian Castellanos upgrade his abilities through the use of Green Gel.
It has been suggested by fans that she might be Ruvik's sister, explaining why she was largely untouched by his violent revenge, but this has been disproven, following the fact that Reborn Laura is created by Ruvik as a tribute to his feelings for her, hence indicating that Laura is Ruvik's sister. It is also shown in Chapter 9 that she is his sister.
In Chapter 14, after Sebastian finds a Missing Person Poster showing Tatiana, she disappears, and does not appear for the rest of the game. This leaves a mystery on her whereabouts after that.