Teppei is a vulgar, short-tempered, violent, and abusive man. His backstory remains unknown to a large extent. He appears in the story supporting his wife in her resentful and abusive behavior towards Satoko and Satoshi, both of whom are seen as a nuisance to exploit for housekeeping and gaining access to money held by their late parents. Theirs is a loveless marriage of convenience: Teppei runs his scams and loansharking with Rina and frequently spends time with her.
As depicted in the arcs, Teppei is a sociopath. He has no moral nor ethical qualms with cheating others out of money and resorting to regular physical and verbal abuse against Satoko. In his violent role, he does not appear as intelligent as his partner Rina; however, he is not a complete idiot. He knows how to play the role of intimidation to debtors and marks to lead them fall further in debt to Rina and himself. He recognizes Satoko's devotion to the memory of her brother and uses this to prevent her from running away or telling the truth to social workers.
Nevertheless, he lacks forward planning, behaves rashly, and is a coward. Though he knows he was not involved in his wife's death, he immediately flees to avoid suspicion and whomever he thinks may have killed her. Similarly, he flees Okinomiya when the yakuza catch Rina. He starts beating Satoko under the assumption that, somehow, there are yet more hidden bank accounts from her parents. In Hinamizawa without his attractive scam partner, he spends the majority of his time playing mahjong and drinking alcohol with his friends while Satoko does all of the shopping and housework.
However, in the sixth episode of Satokowashi-hen, memories of his deaths in other worlds end up affecting him, and making him realize that only a despicable and lonely ending awaits him in the face of his destructive behavior. An encounter with his niece at that moment reveals an unexpected aspect of him, showing altruism and compassion very similar to that of Satoshi, in defending Satoko from being attacked by bullies. Thus far in subsequent episodes of Gō, his change appears legitimate and consistent.
In Sotsu, Teppei then becomes a useful "pawn" for his niece's dark purposes. Throughout all the arcs up to Tatariakashi-hen, he demonstrates a gentle and servile nature to his niece, acting fully in search of redemption for the terrible things he's done, including asking Ōishi for help in helping to protect his niece from the girl's supposed alienation of the village. These actions even extract humanity from Satoko during the events of the Tatari aka shi-hen arc, leading to a momentary crisis of her actions.