Professor Yokoyama is the director of Tokyo General Hospital and the father of Harumi Yokoyama. When the mysterious boy later known as Mars is discovered on a newly formed volcanic island and brought to the hospital without memory or identification, Professor Yokoyama takes responsibility for his care. He examines Mars using a wide range of medical instruments and closely observes the boy's abilities, which clearly lie far beyond those of an ordinary human being. In addition to his clinical attention, he brings Mars into his own home and treats him as part of the household, looking after him whenever he is exhausted or injured by the battles that come to define the story.

His household plays a meaningful part in Mars's early life. Living with the Yokoyama family gives Mars his first sustained experience of ordinary human society, and his perception of humanity is largely shaped by this time in their care. The professor's daughter Harumi teaches Mars many things about the world around him and influences the decision he eventually makes regarding the fate of Earth. Within the narrative, Professor Yokoyama serves as an early protector and caregiver for Mars, offering him a stable and welcoming environment at a moment when his origins are completely unknown and his nature is unlike anything medicine has encountered.

As a professor and physician, his capabilities are those of a scientist and doctor rather than a fighter. He does not take part in combat and has no special powers of his own; his contribution lies in his medical expertise, his careful study of Mars's condition, and his willingness to shelter someone beyond human understanding. Once the conflict with the six guardians begins in earnest, the professor and his family recede from the foreground of the story, and their supportive presence no longer forms part of Mars's everyday circumstances.