There are currently 12 playable classes available in Risk of Rain. Each class features four different skills, along with different stat growths. At the start of the game, only the Commando class is available for use, but additional classes can be unlocked through completing achievements.
One item, the Ancient Scepter, works uniquely on each character, upgrading their fourth ability.
The available classes, from left to right are:
The detailed instructions for unlocking each character can be found here: Character unlocks.