I'll try to elaborate, why I dropped it at Chapter 158 and why he does in fact act like a "Simp" rather than somebody in Love. This is only UNTIL Chapter 158, maybe it will CHANGE in the future.
"Someone who does way too much for a person they like. -> Brian shu the simp: I bought her a car and a whole ass house and we not even dating! " - Urban Dictionary.
He doesn't try to go further than "Rental", she clearly likes him but instead trying to go further, that he wants her as a "Real" Girlfriend, he still rents her or at least he could use the last Rental date as a Confession but didn't and just spent Money on a girl. At this point he's a Sugardaddy with no benefits. All the character development for nothing. Going back to the beginning. The beginning of this Manga was relatable for people and the use of a Rental Girlfriend is fine but many chapters later it shows how dumb the author makes the maincharacter.
Ruka, who became annoying as well even blackmailing him a little( or more? ) and why he didn't break up with her is still unclear. His "date" with Sumi always showed, that their time was the best, better than Mizuhara but she's only there for cheering him up and has no competition at all. Everyone in this Manga is running in a circle. Mami didn't show up since a long time as a real character. Author could explain more, why is she like that? Is there a reason? Why did she choose him? There is no information about this whatsoever, maybe in the future. Chizuhara's character at least recieves an explanation why she behaves like that.
The problem is also his character, all the time you hear him:" Wow, she's so perfect, so beautiful, I am nothing against her, I don't even deserve her". No Character Development at all and the Manga showed, that HE is actlually a good and dependable guy, he made a damn movie, works in that Karaoke(?) Bar. that is why people get furious, because he puts her on a pedestal but himself in the gutter. Does he wants to be her boyfriend / Partner or her Buttler? People thought, that after the Movie Arc he'd finally step up, because he did so much right just to throw it all away. All the time he just spents money, renting her, on her but didn't actually TRY to go out with her for real, as if he's trying to use the Rental as a defensive mechanism, just to say:" It was just Rental ", if something goes wrong. That is the reason why people get angry about the maincharacter and call him a simp, since his behavior is practically the definition of a simp. He literally made a Movie for / with her, her dream. Doing a bit more for somebody you like is normal but not this much.
It seems, that he doesn't love her, it's an obsession.