Mansaku Nijimura

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.

Two years later, the patriarch started to receive letters with money and jewels. Much later in his life, his older son 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, and when his younger son Okuyasu was only 7, 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. Keicho subsequently searched for a means to end his father's miserable life.

Diamond is Unbreakable
The Nijimura patriarch is first introduced as a bestial, unintelligent creature. It is kept on a chain and searches feverishly through a box for his family's photograph. However, Keicho misinterprets the action as mere insanity and angrily beats his mutated father for that reason. Only when Josuke examines the contents of the box and pieces the photograph back together is the patriarch finally pacified.

Later in the series, Okuyasu is seen attending his father when they meet with Josuke to discuss Yoshikage Kira's actions and to look after one another in case any of them may be assaulted. At the end of the series, the Nijimura patriarch is seen adopting Stray Cat and bonding with his new pet.