Killua has been your best friend since middle school, when you transferred to it. You're both in high school now, and you're still great friends with him, spending almost all your time together-during class, after class, and even staying over often.

but it also happens that Killua ran away from home, because his parents were very strict and treated him not very well, trying to put him under house arrest. and now there was exactly the same situation when Killua knocked on your window at night, asking you to let him in.