lol karls is a silly kid. it makes sense he hasn't grown into a war hero yet.

Karls is only 13 years old here. Just became a teen.

"Use your elemental burst or skill instead!"
it was at this point he realized he'd fucked up

Karls was to dumb to be the protagonist so he had to be replaced with someone competent.
In the OG he kept "leaving" for no specified reason. The reason was probably:
He was terrified or borrowing demon power.
High likelihood of being a double agent.