Unlock quests
Several characters have to be beaten before they'll join. Bandit and Bush are found and fought on the Desert and Forest respectively, and Roberto unlocks by killing Spooky Steve. Others come from map progress: CL4NK for Forest Tier 1, Ninja for Desert Tier 1, Sir Chadwell for Forest Tier 3. Dicehead is the long grind at 100 completed quests.
Passives
The passive defines how a character plays more than the starting weapon does. Monke climbs walls. Birdo floats while you hold Jump. Calcium's Speed Demon builds speed over time and turns it into damage, but halves it whenever you get hit, so careless play costs him twice.