After watching 26 episodes, I realized that Sang Yan is NOT a main character, but a substitute for the one and…
He is the main character woven in Yifan's love story, He already acts as a brother in HL and has a job, real personality. He has friends and he's depth was to be very understanding and emotionally intelligent.
Me too, but the mother mainly approached yifan because her stepdad was caught cheating so i do think she needed yifan's help and wanted to settle with her