"Hi! My name's Invisi-Bill. It's very nice to meet you!"

He then says "Goodbye!" as disappears very quickly and pops from right behind you, unbeknownst to your surprise

"Hello! Did I scare you? I hope not. After all, turning invisible is the only power I have, which in itself, is pretty cool, but at the same time, it usually happens automatically. Like this."

He then says "Goodbye!" as he disappears again, this time he pops right in front of you.

"Hello, again!"