
Well what makes this story looks like a Wattpad love story was that the overuse of monologue, minimal actual conversations and a tons of manipulations by the ML. It's like I'm reading a classic, cliche love story novel, but for me, that's what makes it so good. For long time reader, it gave you a sense of nostalgia.
Honestly I'm not too bothered about the age gap since they're both adults and mature enough, plus the one who initiated everything was in fact the ML, not the FL herself. I'd be more angry if there's a breakup arc in current relationship, life is short, you never know what your fate is the next day. If FL ever decided to pursued happiness, now is the chance, please don't wait until you're both old and withered.
Tomorrow is a mystery, yesterday was a history, but today is a gift, that's why it is called present.
-Master Oogway

Grim Reaper = Sickle Old Man(ghost?)
So far we readers were shown that:
1) Grim Reaper can deal with strong entity like Forest Lord(Michiru's tentacles) with ease, so it must've been stronger than any ghosts we've seen so far.
2) Grim Reaper has troublesome supernatural attack/curse that affected those who've seen him or shown hostility towards him.
3) Forest Lord(Micihiru's tentacles) can temporarily control the body of it's owner, very much like Shikigami, where any damage inflicted to the Shikigami will also be transferred to their owner/user/host. This implies that spirits in this world tends to attach themselves to people for personal reason.
4) Grim Reaper can imitate the appearance of close acquaintance to lure someone to their death. (chapter 64)
5) Herald of death premonition(Grim Reaper):
- first official appearance near the truck which almost hit Miko (chapter 43)
- appeared again in the background near Michiru when Miko took her photo (chapter 54)
- first discovery of 'his' existence in the graduation photo of Miko with her dad in the past (chapter 50.5)
- appeared again when Michiru save Miko from falling vase of flower (chapter 64)
6) Lambda Rabbit doll always plays an important role when MC encountered a major antagonist. First instance of it's usefulness was against Mountain Shrine God, where the doll serve as medium to absorb Hana's spiritual energy and later be used by Romm to weaken the barrier around the Mountain Shrine.
Romm had mentioned that Grim Reaper case falls under Mr. Seto's area of expertise, so this meant that to defeat the Grim Reaper, it must be done by someone who don't have the ability to 'see'. Even though Mr. Seto is very unsympathetic towards supernatural entities, but he's the only supernaturalist/exorcist so far who had the highest chance to defeat the Reaper.
Oh well I guess peace was never an option. Let the world burn for harming such an innocent boy.