Just to correct a couple things, Amarant didn't become a bounty hunter to get revenge on Zidane. He simply wanted to have his fight to determine who was stronger.

And for Amarant, going from Security guard to Bounty hunter is no stretch, as he shows a strong desire to fight. Bounty hunter just allows for more mobility then Security guard, and gave him the chance for a fight he desired.

And I used him in battle.

Though yes, It is kind of sad he adds little to the actual story, other then a representation of some abstract concept like the other characters are.