The "Buddy System" will let you switch into any member of the Toxic Crusaders that you want to play as! Wait for your Tag Out Meter to fill up, then switch away!
Simply tap on the designated button to call in a new character, and they'll jump down onto the scene! Your current character will disappear as your Buddy does a lethal jump attack that can follow-up on your combo strings to catch enemies by surprise!
You can also CHOOSE which character you'd like to have jump in! Scroll through the character icons, pick a character that isn't already active on the field, and Tag them in!