Experience

Experience is an in-game point system used to unlock new Characters. It is awarded on the death or level transition screen, and depends on the days survived. Each survived day adds 20 XP. Subsequent games add to the existing experience.

The days survived in Adventure Mode don't count, so surviving 3 days in Sandbox Mode, then going to Adventure Mode and surviving for 5 more days, then dying and return to Sandbox mode, and dying again in Sandbox mode will earn experience only for the 3 days spent Sandbox mode.

Wes, Maxwell, and Webber are the only three characters that can't be unlocked via experience.

Unlockable characters via experience
More than one character can be unlocked at once. For instance, if a player only has Wilson unlocked but earns enough experience to unlock Wolfgang, both Wolfgang and Willow will be unlocked. This means to unlock all characters unlocked through experience (ending with Wigfred), one needs to survive a cumulative 88 days.

Trivia

 * It is possible to unlock all characters (except for Wes, Maxwell, and Webber) by skipping days by using certain console commands.