The Identity of Reiji's Biological Father
Considering Reiji takes after Yuko almost entirely, it's hard to find physical features that one could attribute to a father. However, there are a handful of contenders for the position:- Esemori is one of the top contenders, since their stories are intertwined, they're each other's obsessions, and Reiji does take a lot of personality points from Esomori even though neither of them have met prior to the main story. The timeline as stated doesn't quite add up, however, since he supposedly only discovered Reiji years after the point when he would have been born and he didn't come back to town until his mother's funeral. That, and he states himself that all the drugs he's done over the years have made him infertile. Some fans believe that Yuko's insistence that he's the father meant that Yuko did, around the time of Reiji's conception, leave the town and encountered Esemori, and had a one-night stand with him that he couldn't remember.
- Yuko's Husband. She was already together with him by the time Reiji was born and she didn't mind being with him, but Kazu's unintentional reopening of her wounds along with Reiji looking nothing like his father gave her the perfect excuse to pretend/believe that Reiji couldn't be his child.
- Gen's father. Yuko mentions that she occasionally has sex with him to keep him in line and, from her perspective at least, he had a crush on her ever since high school. It wouldn't be out of place for one of their sessions to have ended in Reiji's conception. Although that would significantly complicate the relationship between Reiji and Gen more than it already is...
- Yuko's father, at least according to Reiji's grandmother. While Reiji believes that Yuko may have lied about it to further isolate him from his family and it's likely that he would have been elderly by the time Yuko could conceive Reiji, it's definitely not a stretch of the imagination that grandfather Kurose continued abusing Yuko into her adulthood, but physically and sexually.
Kazu remained a shut in because he was investing in cryptocurrency.
He was trying to get out of town as fast as he can, so it'd make sense if he thought that crypto investment would have been a faster get-out ticket. But like any inestment scheme, he probably had little luck, and combined with his paranoia about knowing who murdered his father, he just mentally broke.