Special partners, or more commonly known as assist characters, are a set of extra characters you can call on screen to perform an attack. Before each fight starts you will be assigned one assist character. Here is a complete list of who the assist characters are and what they can do. The "Stock" tells you how many times you can use this character per fight.

If you want to select a particular partner, wait until the assist character select screen appears, hold down start and then hold down the corresponding buttons listed in the "Code" part of each description found below. Note that Shadow and Sentinel can only be selected via their codes. And no, I don't know why the Marvel side got 2 more than the Capcom side.
Unknown Soldier appears and fires several shots from his rifle. His orb also fires a beam shot.
Stock: 4
From: Forgotten Worlds
Code: JP


Arthur jumps on and throws three lances. If he gets hit, remember this?
Stock: 8
From: Ghouls 'n Ghosts
Code: JP + SP


Ton-Pooh jumps on and then performs a kick attack which has a large range.
Stock: 9
From: Strider
Code: JP + FP


Anita slowly runs on, points at the opponent and sends a bunch of random projectiles their way.
Stock: 5
From: Night Warriors
Code: JP + SP + FP


Michelle drops in and shoots several triple shots from her assault rifle.
Stock: 6
From: Legendary Wings
Code: JP + SK


Cyclops leaps to the left of the screen and then does a low level optic blast.
Stock: 7
From: X-Men
Code: JP + SP + SK


Storm appears onto the screen and performs her double typhoon move.
Stock: 6
From: X-Men
Code: JP + FP + SK


Rogue jumps down with a punch, then does her upwards multiple punches move.
Stock: 6
From: X-Men
Code: JP + SP + FP + SK


Juggernaut dashes across the screen whilst doing a weaker version of his Juggernaut headcrush move.
Stock: 3
From: X-Men
Code: JP + FK


Colossus moves across the screen whilst doing his shoulder tackle move.
Stock: 5
From: X-Men
Code: JP + SP + FK


Shadow jumps on and then performs a weaker version of his somersault justice move.
Stock: 3
From: MSH Vs. SF
Code: JP + FP + FK


Lou jumps on screen and fires three arrows at his opponent. His orb also shoots a short-range fire blast.
Stock: 8
From: Three Wonders
Code: SP


Saki jumps on and fires a beam at her opponent, that hits several times.
Stock: 7
From: Quiz Nanairo Dreams
Code: FP


Devilot and her crew arrive where your character is standing, and then explode -this can't be blocked.
Stock: 5
From: Cyberbots
Code: SP + FP


Pure jumps in and drops down a lot of dice projectiles along with her cat, Fur. She catches Fur and then leaves.
Stock: 5
From: Capcom World 2
Code: SK


Thor drops down and then fires a thunder bolt from Mjolnir. The bolt can hit multiple times.
Stock: 7
From: Journey Into Mystery #83
Code: SP + SK


Magneto appears on screen and then performs his E M Disruptor move.
Stock: 7
From: X-Men
Code: FP + SK


Jubilee jumps on and throws several small projectiles at her opponent.
Stock: 8
From: X-Men
Code: SP + FP + SK


Psylocke dashes across the screen whilst doing a weaker version of her psi thrust move.
Stock: 5
From: X-Men
Code: FK


Iceman jumps on and uses a more powerful version of his avalanche move.
Stock: 4
From: X-Men
Code: SP + FK


U.S. Agent appears on screen, and then does his charging star move.
Stock: 5
From: MSH Vs. SF
Code: FP + FK


A Sentinel dashes across the screen and will smash the opponent out of the way.
Stock: 4
From: X-Men
Code: SP + FP + FK