[ The bristling is so much more obvious with a set of animal features to go along with it, and Atsumu's bad moods have never been particularly hidden things in the first place. Right now, however, his ears could not possibly be pressed back any flatter against his skull, and his tail ramrod straight pointing out a downward angle. It really couples "nicely" with the furious expression on his face.

Eustace's defense mechanisms make a lot of sense. You face some tough things at a very young age, and you have to learn how to create a shell in order to process that. Atsumu is older than Eustace was when he lost his parents, but he's still young, and he's still struggling to cope with his current reality.

So at that hint of disdain coupled with that "shape up" talk, he's getting agitated. ]

Quit talkin' like you know a fuckin' thing about anything I'm gonna become! You ain't an authority on this shit! None of the assholes here really know what they're doin', or else it'd stop happenin'!

[ He spins (wobbly) on his heel, and tries to stomp off away from Eustace.

The intended effect is dignified outrage, the actual effect is disgruntled penguin on land.

He definitely has nothing on Gen when it comes to teenage outbursts. ]