Ko Soo was jealous of Jin Se-yeon and Seo Ha-joon.
On the episode of the MBC "The Flower in Prison", Yoon Tae-won (Ko Soo) told Ok-nyeo (Jin Se-yeon) now not to hang out with Myeong-joong (Seo Ha-joon).
Yoon Tae-won asked Ok-nyeo about Myeong-jong and she stated it was once a secret.
Yoon Tae-won then said, "Is there anything going on between you two?" yet she did not say anything. Then Yoon Tae-won told her to moveby ability of herself and then she told him that Myeong-jong was a secret missionary.
Yoon Tae-won then told her, "Don't see him. Just don't see him anymore".