We got more information on Taesoo of the past too, he was looking for Mia all the time just wandering around even years after she disappeared, so that's why Heesoo was hesitant to actually express anything towards him, and he knew about her family and arranged marriage situation and made no move in return. This also explains why he wasn't really mad or angry with Mia after they reunited, and was more relieved that she was alive and okay. Then he went on his mandatory military service and came back two years later and into a new apartment on his own, all along being helped by Heesoo. There's a lot of history between these two as adults, and clearly a lot of unrequited, ironically mutual feelings. Probably more to come in the chapters ahead.
Honestly, we're getting all this background info primarily on the heroines of the story, but really we have very little on Taesoo's background and past that doesn't directly involve the girls... Perhaps following the pattern so far, the arc after Heesoo Sunbae will be Hayoon's, and then a proper Taesoo arc, assuming the harem will stop at only 3 members.