At the extraction point, access the character selection menu and enter.

Dare Devil: Left, Left, Right, Right, Up, Down, Up, Down, Start.
Silver Surfer: Down, Left, Left, Up, right, Up, Down, Left, Start.
Submitted by Feingold
Unlockables
Certain character combinations unlock special bonuses.
Agents of S.H.I.E.L.D.:+5% All Resistances: Nick Fury,Spider-Woman,Captain America,Black Widow,and Wolverine.
Assassins:+60% S.H.I.E.L.D. Credit Drops: Ronin,Deadpool,Blade,Elektra,Black Widow,and Wolverine.
Agile Warriors: 10% Reduced Energy Cost: Spider-Man,Spider-Woman, Elektra,Deadpool,Daredevil,and Ronin.
Avengers:+5% Damage: Captain America,Thor,Iron Man,Spider-Woman,and Ms.Marvel.
Bruisers:+15 Striking: Thing,Captain America,Luke Cage, and Ms.Marvel.
Fantastic Four: 20 Health per KO: Mr.Fantastic,Invisible Woman,Thing,and Human Torch.
Femme Fatale:+5Úmage: Ms.Marvel,Storm,Invisible Woman,Elektra,Spider-Woman,Ronin,and Black Widow.
Natural Forces:5% Damage Inflicted as Health Gain: Thor,Storm,Human Torch, and Ice Man.
New Avengers:+5% All Resistances: Captain America,Luke Cage,Wolverine,Spider-Man,Spider-Woman,and Iron Man.
Power Platoon:20 Energy per KO: Silver Surfer,Iron Man,Ms.Marvel,and Thor.
SuperNatural:+5% Health Regeneration: Ghost Rider,Dr.Strange,Blade,and Thor.