Medama Oyaji is the father of Kitaro, the protagonist of Gegege no Kitaro. He originally belonged to a once-dominant race known as the ghost tribe. Before the events of the story, he was a fully formed adult who contracted an incurable disease referred to as the melting disease. While living in seclusion with his pregnant wife, he eventually succumbed to the illness alongside her. After his wife passed away and Kitaro was born from her grave, Medama Oyaji could not rest peacefully. Out of concern for his son, he reanimated by taking the form of his own left eyeball, sprouting a small body and limbs to create the tiny, anthropomorphic figure that he is known as. He stands approximately nine point nine centimeters tall and weighs about thirty-three grams.

Medama Oyaji is defined by his deep love for his son, which borders on doting. He is highly protective and often worries about Kitaro, frequently expressing his care through affectionate nagging or visible anxiety. At the same time, he is a strict father when necessary, disciplining Kitaro for behavior that he considers unbecoming of a ghost tribe member. He is extremely knowledgeable about both Japanese and international yokai, having traveled the world extensively before Kitaro was born. This makes him a valuable source of information and a strategic advisor during conflicts. He is also curious about human culture and, depending on the series, shows interest in modern trends, television, and even smartphones. Despite his small size, he has a strong spirit and a resilient will. He enjoys cleanliness and is well known for his habit of bathing in a small tea bowl, which serves as his personal bathtub.

His primary motivation is the protection and well-being of his son. This paternal drive is the reason for his rebirth and continues to guide his actions. He seeks to keep Kitaro safe from harm and to ensure he grows into a responsible guardian of the boundary between humans and yokai. He also desires peaceful coexistence between humans and yokai, though his attitude toward humanity fluctuates between warmth and skepticism depending on the adaptation.

In the story, Medama Oyaji serves as Kitaro’s constant companion, advisor, and moral compass. He often rides in Kitaro’s hair or nestles in his empty left eye socket. He helps explain the nature of the yokai the group encounters and frequently suggests strategies during battles. While he is not always the primary fighter, his intelligence and experience make him indispensable. He also acts as a bridge between Kitaro and other yokai, maintaining connections with ancient beings such as Enma Daioh, the ruler of hell.

His key relationship is with his son, Kitaro, whom he loves unconditionally. He is also on familiar terms with the recurring yokai allies such as Sunakake Baba, Neko Musume, and Nurikabe. His relationship with Nezumi Otoko is often strained; he finds him opportunistic and irritating, though he occasionally acknowledges his cunning. Across different series, his interactions with Neko Musume vary, and in some adaptations he expresses hope that she might marry Kitaro.

Medama Oyaji has shown subtle development over the various anime series. In the first and second series, his protective and doting nature was especially pronounced. In the third series, he became slightly less overprotective and supported Kitaro’s relationship with a human girl, reflecting a more open-minded attitude. The fourth series emphasized his emotional and comedic expressions, often showing him crying or panicking over Kitaro’s safety. The fifth series presented him as more worldly and trend-conscious, with a keen interest in human entertainment. The sixth series highlighted his curiosity about modern technology and his role as a stern yet loving father who corrects Kitaro’s rudeness. His original form, before becoming an eyeball, has been depicted as a tall, silver-haired man of striking appearance.

Medama Oyaji possesses a wide range of abilities. He has exceptional vitality and can survive being crushed, burned, or flattened, regenerating after a short time. He can enter the bodies of enemies to attack them from within, disabling or controlling them by targeting vital organs. He can transform parts of his body, such as stretching or flattening himself, and can project spiritual energy from his hands for both attack and defense. One of his most powerful techniques is the summoning of a phantom train that can travel through time, though using it exacts a heavy toll on his life force. He also possesses a technique known as reverse mochi killing, which turns mochi into a swarm of eyeballs to smother opponents, but this also shortens his lifespan. In some series, he can fire energy beams from his eyes and create illusions. Despite his small form, his spiritual power is formidable, and he has defeated powerful enemies that even Kitaro struggled to overcome.