hmm there is definitely something between them. A past.

I was seeing it from the angle that he is harassing her. He's her ex- partner/boss. She is probably back at the company to take him on. The way she was latching on to the case in ep 4 made me think that. She probably put ML on to the affair cases because she needs the tenacious ace there. Maybe she even leaks the info to him about her "improper affair". Or her jealous ex-/again colleagues.

I also think FL and SML have their history (bad one). My guess: they were working together, then get into serious relation (evolving even to fiance!?), but they were forced to split due to father/business priorities and the FL was strongly disappointed and hurt (remember her mother sad story which definitely left deep marks on FL). ML is trying now to get back FL, but clearly he has some character/mental issues: up to eps.6 he shows as a frustrated person with dual behavior: formal= calm and official, real = remorseful and emotive, lacking the behavior of a an adult male.

FL has also her hidden agenda: why she accepted to return to Haemu? It will be interesting how she will get her objective while her plans are shaken by her 2 pursuers.