Medama Oyaji originated as an adult member of the Ghost Tribe who contracted an incurable melting disease, causing his physical decay. While secluded with his pregnant wife Iwako, he sold his blood for income. This act caused transfusion recipients to develop ghost-like traits, attracting authorities. After he and Iwako succumbed to the illness, his love for their unborn son compelled his spirit to inhabit his only intact body part—his eyeball—transforming him. His rebirth coincided with his son Kitarō's emergence from Iwako's grave, establishing their bond. He manifests as a diminutive being with a giant eyeball for a head and a small, unclothed body. A simple line represents his eyelid over the pupil, which curves into a downward "U" shape during sleep. A rarely visible mouth exists beneath the eyeball, though he primarily breathes through his pupil. While typically nude, he occasionally wears accessories like a silk hat or robes. His pre-disease appearance remains unspecified in the 1996 series. His personality balances fragility with resilience. Despite his size, he possesses an indomitable spirit and regenerative capabilities, recovering from physical trauma. He exhibits profound devotion to Kitarō, often displaying anxiety or distress during his son's perilous encounters, frequently reacting tearfully. He maintains a fastidious preference for cleanliness, regularly bathing in small containers; within the 1996 series, he uses hosts' teacups for this purpose. His diet includes consuming morning dew, cherries, and plums. He serves as Kitarō's primary advisor due to his encyclopedic knowledge of yōkai, acquired through extensive travels across Earth prior to his rebirth. His expertise encompasses yōkai behaviors, weaknesses, and occult lore. He maintains a unique rapport with Enma-Daiō, ruler of the afterlife, enabling unrestricted passage between Jigoku and the human world—a privilege shared with other reputable yōkai. This access facilitates strategic interventions. His abilities extend beyond knowledge. He demonstrates capacities for bodily possession and environmental manipulation, leveraging his spiritual strength to influence matter. His relationship dynamics include deep reverence for his deceased wife and unwavering paternal commitment to Kitarō's well-being.