I'll just go over the "memory returns" arc. There are one or two arcs that follow. They are also a little crazy.
SPOILER ALERT
ML gets his memory back very abruptly after sleeping for a month. He pretends to be FLs anmesiac husband, but starts treating her like crap. He is constantly at odds with the personality of the ML who lost his memory (amnesiac husband); they did not merge, but became distinctive.
Eventually, ML comes to hate FL, plots revenge on her instead of killing her (she was his original assassination target), and after she finally reciprocates his feelings of love, even after being treated like shit (including -technically- raping her), he leaves her, but not without destroying her emotionally (he kills/destroys a 500-year old tree).
He does this because he believes FL to be the reason his adoptive mother was killed. He was digging the grave in the first chapter to bury her after killing her, to get his revenge on her for causing his mother to die. The truth is, she IS partly to blame, but it's complicated.
It takes a few weeks for FL to finally motivate herself to move forward. She realizes that the man who she considered her "husband" is dead, and she symbolically buries him (she literally has a funeral for him). She also finds out she is pregnant, unfortunately, from the rape.
There are one or two arcs after the "memory returns" arc. Eventually, the ML personalities merge, with the amnesia one being dominant. The smut ends after the "memory returns" arc though. I don't recall anything past kissing.

Just a warning: this story is an emotional rollercoaster. We are barely in the story too at chapter 31.
It's crazy. ~
Also, there will be a period of time you will likely loathe the ML, but hang in there! ٩(๑❛ᴗ❛๑)۶
The weird kid is the absolute champ.