Well first off, despite his young appearance, he's not a human child but a captain in a military of death gods. In his canon, that pretty much means he protects and purifies souls to preserve the "balance of souls". He comes complete with a bunch of destructive and binding magics, a super speed ability, and is an expert swordsman. Oh, and his weapon was sentient before he got here because that's how death god swords are, and his bears the spirit of an ice dragon.

He's very serious, businesslike, and blunt most of the time. Honestly, he has about three canon instances of smiling. There's a reason one of his fandom nicknames is Captain Grumpypants. I suspect there will be some metaphorical headbutting between these two, given how they can both be abrasive. :P