Billy

Billy is a character in the game Karlson by Dani. He was created to be bullied and is almost useless to Karlson (the player character), aside from being a physics object. He also has a brother named Willy see here. Billy will accompany you every step of the way and there's no way where you can get rid of him, as he is completely and utterly invincible. You could probably throw him in lava and he'd be back in a minute or two.

There is speculation that Billy is evil due to the Karlson trailer saying at the end “DON’T TRUST BILLY”It means billy will still dangerous.

The pressure plate puzzle
Billy is essential for puzzles such as the first pressure plate. The player is trapped in a room with Billy and tasked with opening the door using a simple pressure plate. To pass this, you must put Billy onto the pressure plate or it will never open. Billy will immediately follow you, so it is best to sprint to the door before Billy stumbles off the plate and the door closes.

Billy Face & Sound Logic
Billy will make random robot beeping and voice sounds, and will react to stimuli actively affecting him. If he is pushed, he will have a sad face, if he is falling, he will scream, and he will be confused or frustrated about it if the player did that.