Eren might have only showed up because he likes the beach, or someone might have nudged him gently (or not gently) into coming. Either way, he does show up. He's never seen fireflies, of course. He's seen weird glowing shit thanks to Folkmore, but the fireflies aren't exactly like that.
He holds a(n empty) jar in his hands, looking down at it as if he's unsure whether he should actually catch the insects in it. He's a little in awe of them, though whether that actually shows on his face or not is a toss up.
OR
Later, after he's managed to catch some fireflies in a jar by whatever means, catch him sitting on the shore, barefoot. He's close enough that the water comes up around his feet where he sits with his knees bent, arms resting on them. The jar of fireflies is a little ways behind him, safe from the water.
Sorry, was this social hour? He's bad at that. Bully him.
ota
He holds a(n empty) jar in his hands, looking down at it as if he's unsure whether he should actually catch the insects in it. He's a little in awe of them, though whether that actually shows on his face or not is a toss up.
OR
Later, after he's managed to catch some fireflies in a jar by whatever means, catch him sitting on the shore, barefoot. He's close enough that the water comes up around his feet where he sits with his knees bent, arms resting on them. The jar of fireflies is a little ways behind him, safe from the water.
Sorry, was this social hour? He's bad at that. Bully him.