Assist characters are called to the stage when a player grabs the Summon Cube. Unlike Super Smash Bros., where many of them are from obscure/unpopular games that weren't able to merit a playable one, they are all characters from shows the playable ones are from.
These are as FollowsEdit
Cheese (Foster's Home for Imaginary Friends) Goes around screaming on the top of his lungs and synergizes with Billy and Mandy
Coco (Foster's Home for Imaginary Friends) Lays eggs everywhere and syergizes with Vilgax
Dee Dee (Dexter's Laboratory) Dances around and synergizes with Flapjack
Dracula (The Grim Adventures of Billy & Mandy) Starts a dance party making characters dance to synergizes with Johnny Bravo and Captain Planet
Eduardo (Foster's Home for Imaginary Friends) Runs around trying to hug everyone synergizes with Bubbles
Fred FredBurger (The Grim Adventures of Billy & Mandy) Runs around trying to hit people with his frying pan and synergizes with Ben 10 (modern)
Fuzzy Lumpkins (The Powerpuff Girls) Shoots people with his meat gun turning them into meat and synergizes with Father
Gazpacho (Chowder) Throws pickles everywhere that heal you and synergizes with Dexter
General Skarr (Evil Con Carne) Has his army of tanks come and hit you and synergizes with Him.
Gwen Tennyson(Ben 10: Ultimate Alien) Sheilds the character that summons her and synergizes with Blossom
Madame Foster (Foster's Home for Imaginary Friends) Imaginary Friends chase her, damaging foes. Synergizes with Chowder.
Major Glory (Dexter's Laboratory) Freezes other characters and flies away. Synergizes with Buttercup and Numbuh 1
Mandark (Dexter's Laboratory) Goes around shooting lasers and rockets and synergizes with Toilenator
Mung Daal (Chowder) Falls and leaves food that can heal you and synergizes with Grim
Numbah 2 (Codename: Kids Next Door) Goes on a plane from the show flying across the screen twice and synergizes with Monkey and The Scottsman
Numbah 4 (Codename: Kids Next Door) Does a series of punches and kickes. He synergizes With Kevin
Panini (Chowder) She latches on a random character reversing their controls. She synergizes with Hoss Delgado
Schnitzel (Chowder) When hit he'll wobble in place and cause damage (like Wobbuffet) and synergizes with Ben 10 (classic)
Stickybeard (Codename: Kids Next Door) He will get a giant cannon that will hurt anyone and fires a giant gumball. He synergizes with Captain K'nuckles
Val Hallen (Dexter's Laboratory) He fires lightning his guitar everywhere and synergizes with Aku and Samurai Jack
Vreedle Brothers (Ben 10: Ultimate Alien) They fire their guns everywhere hurting everyone and they synergize with Mojo Jojo