My daughter's imaginary friend (long but PLEASE read)?

[Sacha]

My daughter is 18. She doesn't like going out anywhere because it makes her anxious or something. I can't even take her to Walmart without her clinging to me or acting all nervous.Sometimes when she goes out she doesn't talk to anyone there and she'll look utterly miserable, which I sometimes ground her for because it's ridiculous.
She only has 2 or 3 real friends at school and her crush who ignores her she thinks he's her friend too and that they just don't talk much. The boy hates her, from what I've heard. She came home yesterday and said that the kids at school called her ugly and stupid and weird all day.
Whenever I try to get her to make new friends she just looks at the other kids and won't say anything unless they say something to her, then she'll only say 2 or 3 words before the other kid leaves. She told me all of her friends are 16 and 17 and that none of the kids her age like her.
Well, I've noticed her taking extra food into her room at night. I'm fine with her eating in there but I saw her take 2 sodas and 2 sandwiches in there at 11:30pm when she's supposed to be asleep. I thought that maybe she was just really hungry, but she did it again the next day when she got 2 bowls of cereal. She's been talking about this guy named Steven and how cool he is. She says he's always really nice to her and always wants to hang out. I asked her if Steven was real and she looked at me like I had hurt her or something, but she didn't answer me. I asked her mom if she knew about Steven and she said she did and that he's imaginary. Her mom thinks it's cute but I think it's weird and she's probably doing it for attention. She told her closest friend about Steven and then her friend told her that she had an imaginary friend too named Jesse! After that, I really didn't want them hanging out anymore, but I let it slide seeing as how she doesn't really have that many friends in general.
She was off from school last Thursday and she went to sleep on the couch. Well, when I got up to go to work, there was a blanket on the other couch as if someone else was sleeping there. She made a twitter for Steven and made him his own email address. Her teacher says she writes about Steven in some of her essays and that she's stated he's not real except for the last essay she wrote about him, he never said if he was real or unreal. Her teacher said in her class, my daughter will take off her jacket and put it in the seat next to her and pull the chair out a little for Steven. She's put my daughter in the back of the class because she works by herself and the other kids try to cheat. Her teacher said that she had to do a partner assignment that she chose to do alone and she put her name and Steven's name down on the paper because he was her partner.

I don't want her to have an imaginary friend. We talked to her therapist about it and he thinks it's fine but he always asks her about him just in case he's gone away. My daughter has bipolar and major depression and ADHD if that helps any. So, what could be wrong with her and how do I get rid of Steven?