It's clearly meant to be a heartwarming awkward love story, but the MC is incredibly creepy. Despite the review title, I can't quite call him a stalker, because he doesn't follow her around and intrude upon her life, but he does everything else. Has a collection of wedding dresses that he imagines her in. Buys an oceanfront property because it'll have a good view of the sunset that he imagines her in. Builds a house with cringy heart shaped doors and wall panels and built for all the lovey-dovey things he imagines her in.
...despite the fact that they have literally never talked outside his encounters with her being the receptionist. And the MC has an 'awakening' moment in like chapter 2-4 or something where he goes, 'wait, isn't this completely stalker-ish?'. Which he then promptly brushes off and ignores. He's not even sure she knows his name, because she asks it literally everytime he shows up at the guild (which is near-daily - he turns down all obligations and social events just so that he can go on quests so that he can interact with her).
To put it bluntly, he's a simp. A super, ultra, mega simp, that cries with happiness upon realizing she noticed his existence. And no, I don't mean that in the way that he's nice to her or something. People these days will label any male who treats women nicely as a simp, and I really hate that. But no, here the guy is incredibly obsessed with her, and is utterly willing to do anything just to share a few words with her. It's... painfully cringe. Like at one point he notices her unable to eat her regular dish (a fish dish) at a restaurant. The owner remarks that supply is drying up because there's a monster there. So he goes out of his way to kill the monster, despite the fact he's overqualified for the quest subjugation, and is therefore taking the quest away from someone else. And so he can't even get the quest reward. Just so he can provide a fish dish to the girl's favorite restaurant.
The flipside to all that cringe though, is that he's genuinely a good guy, has used his obsession as fuel to become literally the strongest Adventurer in the land. He's cheerful, and kind to everyone, and willing to work hard to protect the average citizen. He's also NOT being overly possessive (not that the author would even have a situation where that was possible, there are literally no other male characters in the story) and stalking her and so on like the stereotypical shoujo male.
Also, in the very beginning he saves a young female dragon, who is regarded as a national protected species, and so he's basically got a little girl wingman following him around, and she's basically the saving grace of this story. She's a good character, and is the only reason there's any progression in both the plot and their relationship. The main female lead is a kuudere, and it becomes clear she also has issues expressing herself, so no doubt without the dragon, there would be no progress until both of them died of old age.
So TL;DR: if you can get past the simp cringy main character's creepy actions, this is a non-harem fantasy romance that sugary sweet heartwarming.