This allows you to use a special character and that character's weapon.

The Colonists who invaded the planet in "Earth Defense Force 5" have rushed to join the fight. Why would a previous enemy join the EDF in defending the Earth? The answer to that question will be clear when you reach the end of this game, so we wouldn't want to put any spoilers here. Let's just say that yesterday's foes are today's friends. And they look just like us humans! (Ribbit.) Anyway, let's put aside our differences for now and fight together!

More details on the official game page of Colonist: "We'll Help, If It Means Saving the Planet"!? ...