Kayoko Uchida is the girlfriend of the protagonist Sunred, and she serves as a stabilizing, grounded presence in a world filled with absurd superhero conflicts. She is an ordinary working woman, employed as an insurance planner, and is often identified as the primary breadwinner in her household. As Sunred has no regular income, she financially supports him, a situation that frequently frustrates her, especially when he wastes money on games like pachinko. Beyond her job, Kayoko also handles the majority of the housework, a role she manages with a high degree of competence. The only notable exception is her lack of skill in sewing, which is highlighted as a rare domestic weakness.

In terms of personality, Kayoko is pragmatic, sensible, and unafraid to speak her mind. She often comes across as the most normal person in the series, interacting with the bizarre events around her with a practical, sometimes exasperated, attitude. Her relationship with Sunred is characterized by frequent, loud arguments over his messy, unemployed, and often rude lifestyle. Despite this constant bickering, which resembles that of a long-married couple, she deeply cares for him. She understands that beneath his violent and lazy exterior, he is a hero who must walk the path of justice. While she nags him about his behavior, her underlying affection and commitment remain clear. For instance, she has stated her desire to make him happy despite his flaws.

Kayoko's role in the story often contrasts with the other characters. She lives with Sunred in an apartment in Kawasaki City, acting as his anchor to normal life. Her most significant and unusual relationship is not with her boyfriend, but with his arch-nemesis, General Vamp of the Florsheim organization. Despite him being a villain, Kayoko gets along remarkably well with Vamp. She frequently consults him for advice on cooking and housework, and they share a friendly rapport. In the second season, she even participates as his assistant in a cooking segment. She is grateful for the existence of Florsheim, as Sunred's identity as a hero has meaning only as long as Vamp continues to oppose him. She also gets along with other monsters, such as Usakotsu, who admires her. Her pragmatic nature extends to them as well; when some monsters struggled to dig a hole, her pragmatic solution was to suggest that Sunred do the work himself.

Kayoko undergoes a subtle but notable evolution. In the earliest chapters of the original manga, her appearance was plainer and less defined, and her personality was initially harsher. Over time, her design was refined and her character softened. The anime adaptation, in particular, solidified her final, more conventionally attractive design from the outset. Her hairstyle also changes during the series, shifting from long to a bob cut. Later in the story, it is revealed that she first met Sunred when she visited his previous girlfriend's house for work, and she fell in love with him then. Her confidence in the relationship grows, culminating in a moment in the finale when she stops a fight between Sunred and Vamp to officially tell Sunred that they should marry.

While Kayoko is not a combatant, she is not entirely helpless. Her abilities are primarily domestic and social. She is a capable household manager, a skilled cook, and a competent professional. She has also been shown to have a combat power rating comparable to that of the weaker monster Usakotsu, which is considered to be at a civilian level. One of her most practical abilities is the social capital she has gained from her association with Sunred. He has inadvertently made her a VIP among the various monsters working part-time jobs throughout Kawasaki City, allowing her to purchase most goods at a discounted price.