Responses
Link to the page where he said it: http://www.mangago.zone/read-manga/junjou_shounen/uu/junjou_shounen-chapter-6/11/
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Link to the page where he said it: http://www.mangago.zone/read-manga/junjou_shounen/uu/junjou_shounen-chapter-6/11/
I thought it was really clear but looking at the comments a lot of people seems to be thinking that they were actually half brothers, remember chapter 6 (page 11), Issei (Kusaka's dad) said that he never do anything sexual with Tsubaki, that their relationship was like one between good friends but their feelings were really strong!
So they didn't even have sex, probably because Issei was married or maybe because they didn't need something physical because their feelings were enough (like Kusaka-kun).
So yeah, Mizutani-kun is son of an unknown person, probably one of Tsubaki's clients, he got pregnant after leaving Issei, if that were not true then the author wouldn't have the need to point out CLEARLY that their relationship was not physical, at all.