I think all the members of TWICE are close, but there are certain members that especially click well with anyone. I think Chaeyoung and Sana are like this in particular because they seem to easily join into any friend group without much fuss. In Chae's case, she's so chill and laid back that it's easy to be friends with her. You don't have to entertain her. She's happy just sitting with you. I think one of the reasons why Mina enjoys being with her is that Chaeyoung isn't the type to force conversation nor does she try to be funny or "interesting". She's just Chaeyoung, and I think Mina appreciates that.
Chaeyoung is very good at "reading the mood". She is my favorite and I feel that she changes the way she interacts with each member. With Mina and Tzuyu she is much calmer and softer which matches their personality and makes them more comfortable With Nayeon and Sana she is louder, jokes more and laughs out loud. With Jeongyeon she acts like a little sister who likes to tease, just because Jeongyeon is the one who likes to play pranks with the members the most
