Right before the Special Partner screen appears, hold 
Start and enter the following codes: 

Magneto 	- Hold Weak Kick + High Punch 
Storm 	- Hold Weak Punch + Weak Kick + High Punch 
Jubilee 	- Hold Weak Kick + Medium Punch + High Punch 
Rogue		- Hold Weak Punch + Weak Kick + Medium Punch + High Punch 
Psylocke 	- Hold Medium Kick 
Juggernaut 	- Hold Weak Punch + Medium Kick 
Iceman 	- Hold Medium Punch + Medium Kick 
Colossus 	- Hold Weak Punch + Medium Punch + Medium Kick 
US Agent 	- Hold High Punch + Medium Kick 
Shadow 	- Hold Weak Punch + Medium Kick + High Punch 
Sentinel 	- Hold Medium Punch + Medium Kick + High Punch 
Lou 		- Hold Medium Punch 
Arthur 	- Hold Weak Punch + Medium Punch 
Saki 		- Hold High Punch 
Ton-Pooh 	- Hold Weak Punch + High Punch 
Devilot 	- Hold Medium Punch 
Anita 	- Hold Weak Punch + Medium Punch + High Punch 
Pur and Fur - Weak Kick 
M.Heart	- Hold Weak Punch + Weak Kick 
Mighty Thor - Hold Weak Kick + Medium Punch 
Cyclops 	- Hold Weak Punch + Weak Kick + Medium Punch