Aru has a habit of cracking his knuckles

every single one of them, even fingers and toes

then he cracks his neck

five times a day

this habit of his annoys Eru very much because he can't stand the sound

bonus:
Aru once tried to crack Eru's knuckles when he wasn't paying attention and even today he still holds a grudge

mainly because Eru let out a weird sound while the others were present so ¯\_(ツ)_/¯