Characters can be leveled up by killing, crafting, hunting, gathering, and moving.
Become the last survivor by actively taking advantage of all the actions you can do in-game!

Weapon Mastery - Increase your mastery by crafting weapons, dealing damage to a survivor or a wild animal.
Hunt Mastery - Increase your mastery dealing or taking damage to an wild animal.
Craft Mastery - Increase your mastery by crafting Weapons, Armor, and Food.
Search Mastery - Increase your mastery by opening boxes, air supplies, gathering, and turning on the security console.
Movement Mastery - Increase your mastery by moving 50m, travel to different areas, and crafting leg armor.
Defense Mastery - Increases your mastery when receiving damage from traps, enemies, wild animals, killing enemies or wild animals, and crafting head armor.

You can mouse over a mastery type to see how to gain Mastery Exp to level it up in-game: