Maybe bcuz they were having a conflict?? You expect them to still have that cheerful and happy vibe??
Chaeri wasn't chill from the start. Throughout the story there were some dark vibes from her past. She seems always ready to beat someone. Slso we still don't know anything about her family except a few phrases from Chaeri herself and I doubt them. Also Seung have family problems similar to Ilmi's. They lead to his aggressive behaviour.
truthh like for chaeri she was literally ready to jump onyu for picking the same drink as her in a convenient store before she realized it was onyu
I do not like how their character changed especially for Chaeri and Seung. Chaeri was feist yet happy and chill at the beginning of the story, and Seung was a happy go luck type and carefree but now they all became dark. Like there's no consistency in their attitude. The only ones who maintained their character is Minu and Onyu all through and through.