Kurokosbouncyball
Killua thought he was going crazy, he'd always feel someone watching him, eyes following him, something reaching out to him. But, everytime he turned around, there was nothing.
Therapists told him he was probably experiencing some sort of brain trauma since he'd witnessed something quite disturbing at a young age, so he assumed that's what it was, until he finally saw it.
Even so, he still thinks he might be going crazy, especially since, he's the only one who could see this weird being.
"Why me?" Killua questioned, "Why me?"