Hashizo Kato, whose real name is Hideki, is a young boy who becomes a central figure in the events of Ai Shite Knight. He received the nickname Hashizo from his older half-brother, Go Kato, after he came to live with him. The family background is marked by tragedy, as Hashizo is the son of Go's father and his mistress. Following the death of both parents in a car accident, a still very young Go made the decision to raise his baby half-brother himself, and the two have lived together ever since.

In terms of personality, Hashizo is depicted as a remarkably responsible and mature child. Due to Go's demanding career as a musician and lead singer of the band Bee Hive, Hashizo often finds himself alone for significant periods. He handles this solitude with a sense of independence, and his closest companion is his pet cat, the intelligent and somewhat grumpy Juliano. This self-reliance does not make him cold, however; he is a kind and affectionate boy who quickly forms strong emotional bonds.

Hashizo's primary role in the story is that of a catalyst. His chance meeting with the protagonist, Yakko Mitamura, sets the series' main plot into motion. He becomes extremely attached to her, and the okonomiyaki restaurant run by Yakko's father, which is called the Mambo, soon feels like a second home to him. This bond is crucial, as it is through Hashizo that Yakko is introduced to his brother Go, sparking the central romance and drama of the series.

His key relationships define his world. The most significant is with his brother Go, who has sacrificed much to care for him. Despite Go being his guardian, Hashizo often displays a level of practical sense that balances his brother's more passionate and sometimes impulsive rock-star persona. His relationship with Yakko is almost that of an adopted family member; he provides a link between her and Go, and her presence brings a new warmth and stability to his life. His loyalty to his cat Juliano is absolute, with the two rarely seen apart, and the cat serves as his constant source of companionship and comfort. The character does not undergo a dramatic personal transformation over the course of the narrative, as his fundamental nature is already one of steadfastness. Instead, his stability and loving nature help to influence the changes in the lives of the adult characters around him, particularly in bringing together Go and Yakko. Beyond his responsible and caring disposition, Hashizo possesses no particular extraordinary or superhuman abilities; his greatest strength is his emotional resilience and his capacity to create a family bond wherever he finds love and acceptance.