Billy the rolling head was a character created by Trumenator for his PCG Get Vin Diesel's Hair Back. In that game Billy was just a small living head that belonged to a race of Billy's that followed the main character by rolling along. Billy does not say much, and he is more of just a pest, but he can sometimes help the player get through tight spaces and can report simple intel he collects.
He later appeared on a jury in The Shadow Realm. (PCG Anniversary Special II)