oh this is SPICY, the gossip the drama!!!! i have to admit im very poorly versed in ritsu's lore... i have rei tunnel vision wrt the sakumas... so im not super sure about the actual lore so ill just do my best w what i know and make a few conjectures! i think ritsus role as a reihi bystander is really interesting and i dont thnk about him enough

pre war year is super spotty in terms of canon so i guess i can just make sweeping statements. in this year we have 16/17 rei and 15/16 ritsu and hiyori... and since this is before rei started going overseas All the time, i imagine the sakubros were on okay terms? pls correct me if im wrong.....

so reihiyo. rei's going out w her more often and becoming a dork in love. i imagine ritsu certainly notices his older brother's change in demeanour at home, him having become a little happier after meeting hiyorin... i imagine ritsu couldnt attend much of school that year and wouldnt have physically Seen rei hanging with hiyori but i like to think reihiyo became something of a famous/popular couple in school because theyre both so so attractive and talented as idols 🤭 the kind that people chatter about! which makes its way back to ritsu! maybe thru mao..??? and also rei's def brought hiyori to his family home before. those two combined prob gave ritsu a pretty clear idea about hiyori, whom his oniichan has been hanging around a lot.... but heres where shit can hit the fan. maybe ritsus like oh, shes just another girl oniichan's messing around with. if shes just a fling then ritsu's seen too many of those already. until he realises how stupid in love rei has become around her and being a dork and grinning daydreamily all the time or whatever and ritsu catches rei doing little affectionate things for hiyori like buying her gifts and leaving them around the house or whatever then he realises Holy shit. my oniichan is Down BAD for this girl!

i think this can segue into really interesting emotional implications for the sakubros and on the reihiyo premise, hiyori could even act as the trigger/ catalyst for the sakubros to split and become very distant in the war era year. the emotional and thematic implications start to go crazy once we have hiyori allied with eichi during the war, and Ritsu helping eichi too... in ritsu's pov i imagine his feelings about hiyori are p complicated. shes at once the girl who brought his brother back to life but also one of the exfine members trying to kill him.. shes the person who's taking his brother away from him and monopolising rei's time, and is also indirectly is the one who sent him overseas... and she's someone "on ritsu's side". its just messed up!!!!

tldr i personally think ritsu had (correct) suspicions but also very complicated feelings toward hiyori