Diluc, Kaeya, and random aches Kaeya experiences every now and then that scare Diluc every time without a fault. you know this random pains here and there and there's a second you think 'oh damn, I'm done for' but that's just your body acting like a bugging PC trying to reboot by smashing itself into the wall.
Kaeya is the buggiest PC to ever happened to be born.
he can be walking with no care in the world, just chilling, chatting, and then he's folded in half, his chest aching, and he's carefully recollecting what the heck it is now. usually it's just a strained muscle, some awkward pose at sleep, some dumb decisions from the old days such as old wounds or scars or such.
and he's just. lying on the ground, waiting for it to pass, unable to talk as his brain is solely focused on pain, and this dumb body-rebooting happens when Diluc is nearby, and Diluc is freaking out.
he genuinely thinks his brother is having a heart attack. then he settles for aneurysm. then he decides against it and thinks it's kidney stones. then Kaeya takes a deep breath, stands up, dusts himself off, and says they should be going.
Diluc: to the Cathedral???
Kaeya, blinking: what for?
Diluc, at the verge of having aneurysm of his own: YOU HAD A HEART ATTACK??? LIKE. JUST NOW???
Kaeya: I think I'd be aware of it, no?
(spoiler: he probably would not. that's the reason why Diluc always brings Kaeya to the Cathedral to get his idiot of a brother checked up. especially if these random aches last for two-three hours and Diluc feels himself greying prematurely)
it's something that stems from childhood, but when he was a kid it was less frequent, but did scare Crepus a couple of times to the point Adelinde was thinking of calling the healer to her master rather to the kid who already returned to ruining the vineyard with Diluc.