Kenjirō Isshiki is a central supporting character who serves as the grandfather of the main protagonist, Akane Isshiki, and her younger sister, Momo. He is a brilliant and self-proclaimed mad scientist, whose innovative work has a profound impact on the world. His most notable achievement is the invention of the Manifestation Engine, a revolutionary device that solved the planet's energy crisis by generating power from the sky. Despite the success of this invention, Kenjirō is not a man of wealth. He maintains a humble research laboratory on the island of Izu Ōshima and pours most of his resources into his ongoing experiments and inventions, a fact that often strains the family's finances and frustrates Momo, who manages the household budget. While he holds patents for his creations, he actively chooses not to profit from them, viewing it as a form of atonement for a mysterious accident that occurred seven years prior to the story's events.

In terms of personality, Kenjirō is an eccentric and hot-blooded individual who possesses an endless well of energy, even at the age of 61. He has little regard for his personal appearance, typically wearing a rumpled lab coat over casual clothes and sandals, with his light brown hair faded to gray on top. His passion for science often overtakes more practical concerns, but his love for his family, particularly his equally energetic granddaughter Akane, is undeniable. He shares a unique "click hug" greeting with her, a small ritual that demonstrates their close bond, even if Momo finds it childish.

Kenjirō's role in the story is that of the classic mentor and creator of the technology that enables the heroes to fight. He developed the Vivid System, a defense mechanism designed to combat the enigmatic alien entities known as the Alone, which threaten to destroy the Manifestation Engine. However, a critical turning point occurs when an accident during the final stages of the Vivid System's development causes his consciousness to be ejected from his human body. His mind is transferred into a small plush sea otter doll, creating his alter-ego known as Uso-kun. His original, motionless body is kept preserved in a refrigerator for safekeeping. In this new, diminutive form, he continues to act as the supervisor and guide for the Vivid System, giving the ignition key to Akane and instructing her and her friends, Aoi Futaba, Wakaba Saegusa, and Himawari Shinomiya, on how to use their Palette Suits to fight the Alone.

Kenjirō's key relationships are primarily with his granddaughters. His dynamic with Akane is one of mutual support and enthusiasm, while his relationship with the more serious Momo is often marked by her exasperation at his spending habits and childish behavior, though it is clear they care for one another. Beyond his family, he works with military officials like Yuuri Shijō to coordinate the defense against the Alone. His scientific curiosity also drives him to analyze the threat, and he plays a crucial role in uncovering the truth behind the Alone and their connection to the accident from his past. His character development is not about emotional growth but rather about the revelation of his past and the consequences of his work. He carries the weight of the accident that occurred seven years ago, which is later revealed to be directly linked to the creation of the Manifestation Engine and the appearance of the Alone. Despite his transformation into a plush toy, his unwavering belief in the power of friendship and his trust in Akane's ability to connect with others, including the antagonist Rei Kuroki, ultimately provides the key to the final conflict, demonstrating his wisdom as a scientist and a grandfather. His notable ability is not physical combat but rather his unparalleled scientific genius, which manifests in the creation of the Vivid System, including its advanced concepts like the "docking" procedure that allows the girls to combine their powers.