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TLDR: Hyun is a childhood friend who was kinda second ML. He kinda gave up his own happy ending by continuing at Dh entertainment full of corruption and sexual favors - all so yijun doesn’t break out into acting by being exploited. Also seems like he had a crush on yijun. And hyun has a fluffy dog that yijun loves.
After yijun is having all this sex we forgot about hyun and his puppy :(
Long story is (and I might be wrong) Hyun is Yijun neighbor friend growing up - he made it big in acting early on with DH entertainment but did so by doing sexual favors(?). Early on he went to see yijun acting class at school cuz either he had a crush on yijun or his acting inspired him when he was down. It seems throughout the story, hyun likes yijun or really cares about him and doesn’t want him to become corrupt the way he did. He also seems to be a cold actor but becomes golden retriever for yijun (which is prob why this new gay actor is surprised to see him like this). Hyun was the 2nd ML. His contract was gonna expire out and not get renewed so he can move on so he can finally escape but in doing so, yijun would be the next actor being signed on. Maybe I forgot it wrong but basically hyun had to choose to set up yijun to sign up for a corrupt company or renew his contract and suffer a bit more. And I think hyun chose to renew in place of signing on so he would never get exploited by DH entertainment.