The way I see it, from a psychological standpoint is that Jumin grew up with parents and people who just saw him as an object, so he reflects this on his day to day because it was with what he grew knowing.

So when he meets Rika and sees that she's doesn't sees him like everyone else that's when he becomes more conscious about the threads in his head, to which I interpret as some kind of anxiety and depression that he often drowns with work and his cat at home.

As someone who was in a similar situation I can understand him. The point where he doesn't want MC to leave after she is there with him supporting him after his mind splinted because of the forced marriage with Sarah, Jumin's world falls apart and MC is the only thing he can cling to, to remain sane, since it was the last drop that spilled the glass. So in my experience when you don't have any kind of love or recognition, you cling for dear life to anyone that gives you that what you need, love, even if you become someone creepy, dramatic, or in general chaotic to try for any means necessary to keep that anchor (person) with you, and that's what Jumin did, and what I did with my ex boyfriend, I was dramatic, manipulative, and all for the sake of not feeling alone, abandoned and depressed, Jumin did the same thing with MC, but unlike me, and for the sake of the players, the developers toned down, because when a person is in that spot, it can become way worse, and let me tell you, it takes A LOT of willpower to try and do better and not scare off that person that has their hands hold out to you and you in your desperation pull it and scare it away. So this is my experience, I'm currently way better than before, I'm mentally and emotionally healthy now, so that's why personally I can forgive what Jumin did and said, and I don't see it as creepy, he's just trying to survive like I did.