This will cover the characters that can be unlocked through special means. All of their abilities, as well as how to unlock them, is displayed under the character's name.

The Collector
Weapon: Nodes
Abilities: Beam Deflection, Laser
How To Unlock: Collect All Red Power Bricks

Stan Lee
Weapon: Bow and Arrow
Abilities: Speed, Sharp Shooter, Sonic, Explosive, Laser, Shield, Beam Deflection, Repair, Transform (Iron Stan, Excelsior Hulk, Stanbuster)
How To Unlock: Rescue all Stan Lees in Peril