When you reach Lv.5 in Borderlands, you character gets a special action skill that you can do stuff with it.
Lilth: Phasewalk
When activated, Lilith becomes invisible for a short time and when the phasewalk ends, she releases a phase blast that damages nearby enemies. But you can end phasewalk before by melee attacking.
Roland: Scorpio turret
When activated, Roland deploys a turret that can damage nearby enemies, but the turret lasts for a short
time and then takes quite a time to recharge.
Brick: Berserk
When activated, Brick becomes very pissed and becomes invulnerable for a short time. While under the
effect of it, he can make very big melee damage but he cannot use weapons while it.
Mordecai: Bloodwing
When activated, Mordecai releases a bird (or is it an eagle, I don't know) that can kill or just damage
an enemy.
It would work as a player type.