Nijimura's Father

The father of the Nijimura family is a side character introduced in Part IV: Diamond is Unbreakable.

History
Twelve years prior to Diamond is Unbreakable, the Nijimura family's matriarch died, leaving the father in depression. On top of that, Japan had been going through a tough time economically and the family soon fell into poverty. The father constantly abused his sons as a result.

However, two years later, the patriarch started to receive letters with money and jewels. Much later in his life Keicho would realize his father had sold his soul and heart to Dio Brando, who, at that time, was scouring the globe for a stand user capable of recreating reality.

Dio, however, distrusted the Nijimura patriarch and implanted a seed of his undead body within him. When Jotaro defeated Dio, the seed burst, causing the father to slowly mutate into a monstrous form, and later lose his ability to speak. This form would automatically heal any wounds inflicted.

Diamond is Unbreakable
The Nijimura patriarch has mutated into a bizarre creature due to Dio's spores when Okuyasu was only 7. It is revealed at the end of the series that Gatta has gone to live with the patriarch of the Nijimura family after Yoshikage Kira's death.