
Taejun didn't like Han sleeping with Yoon. He wanted to. He was checking up on Yoon and telling Han things. He even took the ring Yoon gave him. Yoon was aware of it but Han didn't seem to care one way or the other. That's why Yoon replaced him as Han's Secretary. Wasn't nothing he could do because Yoon was smarter lol

I can see your point but to me (and this is just my opinion) it didnt seem like he was trying to be hurtful on purpose he was jealous of yoon and a but taken back by han's real personality, and i think he kept tabs on yoon cause to be honest yoon was a little unpredictable. but then again i think the beauty of this story is that each person can choose to interpret it how they want since nothing is confirmed
Taejun is a cinnamon roll and he did not deserve to get dragged by those 2 assholes drama