There are a series of unlockable characters that the developer shared on Twitter. Codes below. There is one additional remaining.
Press code input at title screen and then go to options to select.
1. Copper - up down left right select start b a
2. Eric Bischoff - up up down up up down up a
3. Arlo - up down up down a right left select
4. Howard Phillips - up b down b left b right b
5. ?????
