yuer

Basically the father and his sister (Hanae) has an incest relationship and she gave birth to Ranzou (I guess that’s why he’s not that smart), after that the father married Ikuro’s mother who’s also Hanae’s close friend. She unknowingly adopt Ranzou then went crazy after the truth is revealed. The father seems to be in love with his sister that’s why he fucked Ranzou as a replacement for her and even gave him all of the inheritance as he’s his beloved child

Ikurou’s father (Seizou) had a younger sister (Hanae). they fucked and Hanae became pregnant to then give birth to a child, and that child was Ranzou. Hanae told Ikurou’s mother (Mitsue) that the father is unknown and Mitsue, being Seizou’s wife, decided to kind of adopt Ranzou out of pure compassion. later, after Ikurou was born, Mitsue learned that not only Seizou is sexually abusing Ranzou but that Ranzou’s father is actually Seizou. that made her mental health deteriorate completely.

wdym hes not that smart he was literally choked as a child

I think ranzou turned out like that because of the incest as well as the physical and sexual abuse he suffered when he was a kid.

those can definitely be some factors but choking a toddler let alone on the brink of death can lead to brain damage