To unlock new Heroes, you need to save up your Command Points and you can then purchase new Avengers. Remember though, you can only take two Avengers into battle, so there is little point in trying to get them all, except for completionists. You can see a list of Heroes below, along with their class and Command Point cost:
Iron Man - Blaster
Black Cat - Infiltrator
Black Widow - Infiltrator
Captain America - 130 Command Points - Tactician
Colossus - 25 Command Points - Bruiser
Cyclops - 15 Command Points - Tactician
Daredevil - 85 Command Points - Scrapper
Dr. Strange - 25 Command Points
Hawkeye - Command Points
Hulk - 130 Command Points - Bruiser
Human Torch - 40 Command Points - Blaster
Invisible Woman - 15 Command Points - Infiltrator
Iron Fist - 15 Command Points - Scrapper
Kitty Pryde - 40 Command Points - Infiltrator
Luke Cage - 25 Command Points - Scrapper
Mr. Fantastic - 40 Command Points - Tactician
Ms. Marvel - 15 Command Points - Blaster
Nightcrawler - 65 Command Points - Infiltrator
Phoenix - 65 Command Points - Blaster
She-Hulk - 15 Command Points - Bruiser
SIF - 25 Command Points - Scrapper
Spider-Man - 130 Command Points - Infiltrator
Spider-Woman - 40 Command Points - Scrapper
Storm - 65 Command Points - Blaster
Thing - 40 Command Points - Bruiser
Thor - 130 Command Points - Bruiser
War Machine - 65 Command Points - Tactician
Wolverine - 130 Command Points - Scrapper
How to get Free Heroes:
You can get one free Hero in Avengers Alliance, and this comes after you beat the first chapter in the game (after the first five missions). Other heroes you have to buy using your hard-earned Command Points.
The Heroes in Avengers Alliance are split up into Classes. These are the Blaster, Bruiser, Scrapper, Infiltrator and Tactician classes, which are outlined below:
Blaster: This class has guaranteed critical hits against Bruisers, ignores Bruisers defenses, and takes reduced damage from Bruisers.
Bruiser: This class gets Enraged when hit by the Scrapper class which deals extra damage on the next attack and takes less damage while in this state. It also has increased accuracy against Scrappers.
Scrapper: Scrappers receive an extra attack when fighting Infiltrators, and take less damage from Infiltrators.
Infiltrator: This class counter-attacks whenever it is attacked by a Tactician, and also has increased accuracy against them.
Tactician: These get an extra turn when attacking Blasters and take reduced damage from them.
The class system can be summarised as follows:
Blasters are strong against Bruisers, who are strong against Scrappers, who are strong against Infiltrators, who are strong against Tacticians, who are strong against Blasters.
There are also some 'Generalists', who do not have an assigned class. They are equally strong (and weak) in numerous areas.