The girl is a shut in. She was a happy girl until she started realizing a truth. And she has an anxiety about a monster.

AFter they fall in love, she becomes aware that she is a fake(split) personality that the original girl developed. But they didn't switch. One was totally in control and the other dormant and no one noticed. So the original still has a little kids mentality when she resurfaces.

shi ni itaru yamai?

yeah thanks. I was a little off on her being a shut in exactly. Seems I never listed it as having read it so I was looking in the wrong place(and google just kept giving me elfen lied results).