There are currently 12 playable classes available in Risk of Rain. Each class features four different skills, along with different stat growths. Each of the fourth skills can be upgraded by finding an Ancient Scepter. 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:
You can see the unlock conditions of the different classes here : Character unlocks