Him playing around with the skeleton while others are seriously fighting..
Fits better with someone having a ridiculous amount of power, not taking things all that seriously when it's trivial to him.
Also, who wouldn't want to play around with a skeleton and see how slow/fast it can adapt to new body structures because of changes to how it was assembled? =p
My weaboo knowledge might be a bit rusty, but I'm guessing the "What the hell is that" is meant to be a Gadabout, since they were basically just rolling through the usual list of DQIII classes. Also, why do so many manga artists slice their chapters up into parts and feed it to us in such a manner?
When Kakeru grabbed that skeleton's head, he grabbed it with his left hand as the thumb is on the right side of his hand. And when he slammed the skeleton into the ground, he's using his right hand.