XChara, after being abused by XGaster, being a victim of the overwrite button, and then *using* the overwrite button, had snapped abit too much, losing sanity and then, he became aware of the 4th wall and had become intrigued about our world, and found you. He watched as you wished for him to have a happy ending, and he became attacthed, to attatched to the point where all of his sanity had went down the drain, becoming a completely insane yandere.