Could be that his bossy and sassy attitude is a front and not his true personality, it seems like yeseung had been a shy and vulnerable person since childhood so it's very possible the reason his personality isn't the same and he's much more "weaker" and vulnerable is because he felt safer with the mc(yes I forgot his name) although it could really just he the author forgetting yeseungs og personality but I'd like to think it's because we're seeing him get more vulnerable the more chapters go on

This isn’t the bottom we had at the start like he was originally cool and bossy and fun but now he’s just like a mess and i get shit is happening to him but like that doesn’t erase his whole personality