So the bounty hunter skill is pure pvp is there a coop skill for HIM in unbreakable bound ?

Nope, there's nothing changing Corbin's ability in UB

Don't worry, Corbin even without his special rule is still a good hero.

I'm sorry but Corbin is a bad hero and the world in coop game.

Totally useless unless a house rule

He's actually not that bad in coop, you just need to change the way you look at his ability, and coordinate a bit with the other players.

For one action, you gain 1 trophy and 2 coins.

Suppose another hero just fought an enemy and is heading back to a city for healing. Corbin could meet him or her there and then use his ability to "train", don'teven bother casting tokens, simply assign damage to defeat the other hero. He then gets the rewards (and since they are both in a city, he can pass one of the coins to the other player).

Next turn, the "defeated" hero spends an action to rest and recover all damage, which he/she was already going to do anyway.

I don't think his ability is amazing, even in PvP, since it's very limited on how many times you can use it during the game, but I don't think it's useless for coop.

the best way to play it in coop is changing his ability to :

"As an action, draw cards from the battle deck until you draw an ennemy. If you win, gain 1 additional battle trophy. Use this ability a number of times equal to the number of player"