Basically what happens is that ML killed his father and brother and also the fiancée, the mother is still alive and FL moved out of the house ML got for her cause she thinks that ML lives happily with the fiancée since he never responded to her letters and she sees him after 3 years outside of where she works, it’s pretty awkward and all, later during the day FL offers some ice cream to ML and congratulates him on his engagement, ML gets kinda mad/ frustrated and he leaves her but he makes her come back in the country where he is through a job offer.
ML wants her to move in with him but she doesn’t want to, she goes to his house to talk and stuff and they end up kissing and dry humping and when FL goes back to her hotel room there is a robbery (planned by the mother of ML). FL ends up admitting her feelings at some point and they kinda fuck a lot after that. In the meantime she found back her letters and she gives it to him sooooo much later like the FL was so insecure about it and ML thought that FL didn’t think about him at all during the 3 years they were apart but everything gets resolved and they fuck again lol
then the mother comes back and ML was planning to kill her but the mom said to FL what ML did to his family but FL isn’t bothered at all by this and she even threatens to kill the mother herself lol and then the ML proposes and they get engaged (she gets pregnant in the side stories)
Kinda skimmed on some details but i hope i was able to answer your question ^^
Here ! ^^ it’s starts from chapter 74 (ML merting FL for the first time) enjoyyyy :)
https://eternalune.com/novel/a-butterfly-through-the-mist/

The manwha feels so rushed compared to how well written the novel is, everything is going so fast but omg i am so excited for the reunion the ml is quite literally obsessed with herrr