If you don't want your antagonist to be a person, you could always make it a situation or organization or an idea or something, if writing about shadowy presence doesn't sound appealing. I mean, as long as if gets in your characters' way and impedes their progress, it could be anything, even your character's self-perceptions. (I think. Someone kick me if I'm wrong.)

I'll probably just choose my second main character, whom my protagonist considers to be his most tangible enemy at the beginning. Maybe. I'm still deciding.