Yechan is already grown up, is just that he acts that way to people who are close around him. Take it as an example that we may act more ridiculously around our family rather than people outside in public. If you see how he first greets his lover's sister and mother + other people he's formal but not all the time that bad <( ̄︶ ̄)>
I hope this is a somewhat summary for how he acts like?? Idk my words aren't so great ( ̄∇ ̄")

I’m ch31 in,,, can anyone summarize yechan’s character growth for me?
don’t hate me but I can’t do this,, he’s a grown ass 20 something :’’’ i get immaturity and allat but this is,,,,,