Assist characters can be called by pressing the Assist key. There is a cool down period between calling assists. Some of the lost crew members will be assist characters, each with unique attacks and utility.
Assist characters typically start at Level 1, and can get to level 2 once you reach level 6. They can reach up to Level 5 with repeated use, especially while in combat(you don't have to hit an enemy, but it must have at least 1 enemy at the current map). Maximum assist level is limited by a function of the player character's level, and level 5 assists are available at level [18-19] or higher. Level 6 can be reached when the character's player is level 20.