okay so i barely remember this manga but it's driving me crazy that i cant find it
the plot was that one character assaulted a second character while they were in school and then the character who did the assaulting became a bus driver later in life. his victim is a passenger on his bus and rapes him.

it should be at least ten to fifteen years old because i read it around ten/fifteen years ago

https://www.mangaupdates.com/series/wjc681a/sono-kuchibiru-ni-yoru-no-tsuyu

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ahhh you found it, thank you so much!!