A-5, also known as Smileyman, is likely one of the first entities that you will encounter in the game.

Smileyman appears to be a basic, pitch black smiley face and opens its mouth to talk occasionally, Smileyman is shared across other rooms fangames, one of which is Confusing Rooms, while the other is Black Rooms (Unfinished).
Smileyman, in an update, has gained captions above its head.
Smileyman ambience actually originates from LSD Dream Emulator. It just has a massive flange effect on it.