Certain character combinations unlock special boosts.

Boost	How to unlock
Agents of SHIELD	Captain America, Nick Fury, Spider-Woman, Wolverine
Agile Warriors: 10% Reduced Energy Cost	Spider-Man, Spider-Woman, Elektra, Deadpool
Air Force	Human Torch, Ms. Marvel, Storm, Thor
Alternate Identities	Ironman - War Machine, Ghost Rider Western, Spider-Woman - Secret Wars, Ms. Marvel Ventura, Thor - Beta Ray Bill
Assassins	Blade, Deadpool, Elektra, Wolverine
Avengers: +5% damage	Captain America, Thor, Iron Man, Spider-Woman, Ms. Marvel
Bad to the Bone	Blade, Ghost Rider, Luke Cage, Wolverine
Bruisers: +15 Striking	Thing, Captain America, Luke Cage, Ms. Marvel
Classic Avengers	Black Panther, Captain America, Iron Man, Thor
Dark Past	Blade, Elektra, Ghost Rider, Spider-Woman
Defenders	Dr. Strange, Iceman, Luke Cage, Silver Surfer
Double Date	Black Panther, Invisible Woman, Mr. Fantastic, Storm
Fantastic Four: 20 Health per KO	Mr. Fantastic, Invisible Women, Human Torch, Thing
Femme Fatale: +5% Damage	Ms. Marvel, Storm, Invisible Woman, Elektra, Spider-Woman
Flashback	Iceman in Snowy Form, Captain America in WW2, Daredevil in Original, Ghost Rider in Original, Iron Man in Classic, Orig. Ms. Marvel, Classic Wolverine
Martial Artists	Black Panther, Captain America, Daredevil, Moon Knight, Nick Fury
Marvel Knights	Black Panther, Daredevil, Dr. Strange, Luke Cage, Spider-Man, Moon Knight
Marvel Royalty	Black Panther, Dr. Strange, Thor, Storm
Natural Forces: 5% Dmg inflicted as Health Gain	Thor, Strom, Human Torch, Ice Man
Natural Leaders	Captain America, Dr. Strange, Mr. Fantastic, Nick Fury, Storm
New Avengers: +5% All Resistances	Captain America, Luke Cage, Wolverine, Spider-Man, Spider-Woman
New Fantastic Four	Ghost Rider, Luke Cage, Spider-Man, Wolverine
Power Platoon: 20 Energy per KO	Silver Surfer, Iron Man, Ms. Marvel, Thor
Raven Ultimates: +6 body, strike and focus	Deadpool, Colossus, Moon Knight, Thing
Supernaturals	Blade, Dr. Strange, Ghost Rider, Thor
Think Tanks: +15% Max Health	Dr. Strange, Mr. Fantastic, Spider-Man, Iron Man
Weapons Specialists	Blade, Captain America, Deadpool, Elektra, Nick Fury
X-Men	Colossus, Iceman, Storm, Wolverine