A Boss is a character that is battled near the end is often not playable.
ClayFighter[]
Near the end, N. Boss is battled, his name being a jab at Street Fighter's M. Bison. He is known for being very hard. The only way to play as him is through a glitch.
C2: Judgement Clay[]
Each character has their own boss, mostly clones.
Clayfighter 63 1/3[]
Each character has another character as their rival.