
My bad, it's not that the blonde guy was the sponsor but he was the one who forced him to sign the contract while Sihyeon was drugged. The actor wasn't the sponsor but he was there during the incident that broke Sihyeon. Source: https://www.novelupdatesforum.com/threads/monochrome-rumor-%EB%AA%A8%EB%85%B8%ED%81%AC%EB%A1%AC%EB%A3%A8%EB%A8%B8.142201/page-4

I'm not sure if it's still valid, no mentions of that current contract in the spoilers, but I'm sure Hajin won't take it lightly if it still is. The ex bf still wants to ruin Sihyeon. At one point he succeeded, I believe he pushed Hajin and his sister off the stairs and Daejun saw it, but his demise is quite satisfying and he bit his own butt by sending his own scandal to the media instead of Sihyeon'#s
Wait...the sponsor of Sihyeon is the actor right? The male actor from the drama where he acts as a loan shark. It was hinted at the last chapter of season 1 of I remember correctly.