Oh, you are a shipper. Explains the taste in writing. This shit is right up your alley.
Tbf it's not about ship bias or not.
I had the audacity to reread Domekano (i'm now on ch70 out of 276) and can tell Rui was being foretold to be some sort of rebound for Natsuo. Yea, there could have been very conflicting feelings for her because she was his 1st time and she was the 1st to show affection towards him despite of starting at odds due to Rui being an antisocial, but the pattern was always that Rui was some sort of rebound or backup plan for him and him trying his best to see her as just an imouto, and having his moments of deviation because she was his 1st time and men are horny and such. But from what i'm seeing with clarity, the thing was that Natsuo was always more of a simp for Hina and was too eager to become an adult so she could take him seriously (a reminder Natsuo agreed to have sex with Rui because 1st he got rejected by Hina and 2nd, he was desperate to become an adult),
despite of the fact that she was engaged with Shuu at that time and him and Rui had a hand on their breakup. My opinion can get expanded further as i resume the rereading.
Domekano's spice was the telenovela romance, but the core theme was Natsuo having to learn the hard way what does mean to be an adult, given he was the hotheaded type of person. And it was waaaay more than just being 18yo so you could rail your hot teacher and become le NTR bvll.