From the rulebook:

Tired of going it solo in a duel? Is one lone fighter just not enough? Strikers are the answer! Strikers allow you to call in your favorite characters during a duel and utilize their attack or support abilities. They shine the most in Tag-Team games, where you can constantly swap in and out your characters to take advantage of a whole gamut of new coordinated combos.

Strikers is an expansion to both BattleCON: War of Indines and BattleCON: Devastation of Indines. One of these two games are required to play with this expansion.