Her.

There's another girl in my head and she doesn't want me to eat. I didn't want to tell Liam about it. I feel crazy, like, dissociative identity disorder crazy. She's my biggest source of comfort and my loudest critic. Liam won't let me use pronouns when I'm talking about her, because she's just "the section of my brain I've partitioned off to help me deal with my guilt and perfectionism," but I think it's easier to refer to her with a pronoun.