Kokichi Muta, known far more widely by the name Mechamaru, is a second-year student at Kyoto Jujutsu High whose entire existence is defined by a profound physical tragedy. He was born with an extreme Heavenly Restriction, a binding vow placed on him at birth. In exchange for a body so severely crippled that he cannot walk, sit up, or even bear exposure to sunlight, he was granted an immense reserve of cursed energy with a range that covers the entire Japanese archipelago. His real body is so fragile that he lives submerged in a special liquid inside a dark, enclosed tank, experiencing constant pain. To participate in the world of jujutsu and interact with others, he creates and remotely controls a cursed corpse puppet he names Ultimate Mechamaru. His self-loathing is so deep that he informs his parents he has died long ago and adopts the identity of Mechamaru, effectively erasing Kokichi Muta from his own life.

Kokichi's personality is shaped primarily by his isolation, bitterness, and deep envy. He initially displays hostility toward others, most notably toward the sentient cursed corpse Panda, whom he openly despises. He projects his own self-hatred onto Panda because he resents the fact that a non-human creation possesses a functional, healthy body while he, a human being, is trapped in a broken shell. This bitterness masks a profound loneliness and a desperate desire for connection and acceptance. His core motivation is to finally experience life as a normal person, standing alongside his peers without being treated differently. He becomes fixated on the idea that the only way to achieve this happiness and to be accepted is to acquire a healthy physical body.

This desire drives his most critical role in the story. Kokichi secretly makes a binding vow with the cursed spirit Mahito and the schemer Suguru Geto, agreeing to act as an informant for the cursed spirits, feeding them intelligence on the jujutsu world. In exchange, Mahito promises to use his Idle Transfiguration technique to heal Kokichi's real body. This act of betrayal is a pivotal turning point in the series, as the information he provides helps orchestrate the devastating Shibuya Incident. However, immediately after Mahito fulfills his part of the vow and Kokichi experiences a healthy body for the first time, the deep-seated desire to protect his friends overrides his selfishness. He instantly betrays Mahito and Geto, transforming his Mechamaru puppet into its ultimate form to fight them.

Kokichi enters the cockpit of Ultimate Mechamaru Mode: Absolute, a giant humanoid combat form fueled by seventeen years of accumulated cursed energy. He unleashes a devastating arsenal of energy beams and homing missiles against Mahito. He even replicates a Simple Domain barrier technique that he learned by observing his friend Kasumi Miwa, using it to briefly neutralize Mahito's Domain Expansion. Despite this overwhelming firepower, the cursed spirit survives the assault and, in a moment of vulnerability, touches Kokichi's real body, killing him instantly with Idle Transfiguration.

Even in death, Kokichi's actions protect his friends. Before his final battle, he had prepared dozens of Mini-Mechamaru units programmed with his consciousness and tactical information. These units guide the surviving Kyoto and Tokyo students through the Shibuya Incident, sending the weaker sorcerers away on distant missions for their safety and relaying crucial warnings about the enemy's abilities. His development becomes tragically clear at the very end. He realizes that the healthy body he sacrificed everything for was never the true goal; it was merely a means to an end. What he really wanted was the love and camaraderie he felt for his friends, particularly Kasumi Miwa. His greatest flaw was that he could not accept that his friends already cared for him exactly as he was, regardless of his physical condition. His final message to Miwa carries no regret, only a gentle wish for her happiness.

Kokichi's notable abilities are entirely rooted in his Heavenly Restriction. The extreme fragility of his real body grants him an unprecedented range and capacity for cursed energy manipulation, allowing him to control multiple puppets across vast distances simultaneously. His core skill is Puppet Manipulation, through which he operates the Mechamaru line of cursed corpses. His ultimate technique is Ultimate Mechamaru Mode: Absolute, where he pilots a massive combat mech with immense physical strength and a wide array of long-range energy weapons. He also demonstrates proficiency in Simple Domain, showcasing his keen observational skills and intellectual ability as a sorcerer.