Responses

It does. I think there has to be an aspect of 'not-incubus' as the other dude mentioned getting married i.e. that its a weird and uncommon thing, but not impossible.
I don't know what the supernatural rules will be, but its almost a given that there will be something that ties them together without the incubus power.
So he is an incubus which means he has sex with women as they sleep/in their dreams. He's been haunting Jaehee.
Theory 1: He's cast hypnotism on the office and he actually hasn't been 'team leader' for very long. Only the length of time she has been dreaming about him. Everyone 'remembers' that he joined the company 3 years ago, but there is no record of him. He also doesn't know Jaehee's dating history in the company or realize what the male co-workers are gossiping about. (Which if he had been around for 3 years he would have.)
Theory 2: The bracelet is working. It's making Jaehee less susceptible to the hypnotism (part why she is noticing that Mr. Seo hasn't been around that long) and Mr. Seo isn't getting into her dreams.
Note that when the power outtage happens and Mr. Seo acts cold is also when Jaehee remarks that she isn't dreaming of him. He mentions that he was on edge himself.
Mr. Seo hypnotized Jaehee to get her to tell him about her dreams. His eyes flashed red as he suggested that she could tell him anything. Her eyes flashed red when she said she had been dreaming, but then when she asks to touch his chest, her eyes are NOT red.
Theory 3: Jaehee shook off the hypnotism and her propositioning him is her real desires. She is actually attracted to Mr. Seo... not just being hypnotized into being attracted to him. (His face is really her type.)
Conjecture: Mr. Seo falls in love with Jaehee because the power is weakened/not working and he finds out that those were her real feelings. However Jaehee will be confused and think that all her attraction for Mr. Seo came only from hypnotism.