There are currently 10 playable classes available in Risk of Rain. Each class features 4 different skills, along with different stat growths. At the start of the game, you will only have the Commando class available for use, but you can unlock additional classes through completing achievements.
The available classes, from left to right are:
Characters
Commando
Enforcer
Bandit
Huntress
HAN-D
Engineer
Miner
Sniper
Acrid
Mercenary