Kayoko Uchida is Sunred's girlfriend and the primary financial provider in their relationship through her job at an insurance firm. She regularly gives Sunred money, which he often spends on pachinko, frustrating her. Despite frequent arguments and Sunred's unemployed, rude lifestyle, Kayoko remains committed. Their interactions resemble those of a long-married couple, characterized by bickering but underlying care and affection, as she understands Sunred deeply cares despite his abrasive exterior.

Kayoko maintains an unexpectedly positive relationship with General Vamp, leader of the antagonistic Florsheim organization. She often consults him for housework and cooking advice, engaging in friendly conversations with more enthusiasm than she shows Sunred. Their rapport includes co-hosting "Vamp & Kayoko's Quick Recipes" segments during the anime's second season.

Her character design evolved during the manga serialization, starting plain and chubby before transitioning to a more conventionally attractive look, which the anime adopted from the start.

Kayoko demonstrates practicality and a no-nonsense attitude. She interacts matter-of-factly with Florsheim members, such as expressing concern about the Animal Soldiers getting dirty while digging Sunred's grave in a playground sandbox and suggesting Sunred dig it himself. This pragmatism helps her maintain normalcy amid the absurdity of Sunred's conflicts.

Approaching 30 years old, Kayoko fits the "Christmas Cake" archetype in Japanese media but is noted as fortunate to already be in a relationship despite this cultural trope. Her occupation, relationship status, and fundamental personality remain consistent throughout all media.