Kokichi Muta, operating through his proxy Ultimate Mechamaru, suffered a severe Heavenly Restriction from birth. This left him without a right forearm, paralyzed and insensate below the waist, and with skin hypersensitive to light and air, causing constant needle-like pain. Confined to a dark cave within a life-support bathtub, he lived in isolation. His condition granted him immense cursed energy reserves and output, enabling manipulation over vast distances. He interacted remotely using his Innate Technique of Puppet Manipulation, controlling cursed corpse puppets. His primary proxy was the humanoid mechanical puppet Ultimate Mechamaru, which attended Kyoto Jujutsu High School in uniform, participating in classes and missions. Other puppets included reconnaissance units and combat models with weapon systems. Through Mechamaru, he utilized abilities like cursed energy blasts ("Ultra Cannon"), limb transformation into blades ("Sword Option"), defensive shields ("Ultra Shield"), and propulsion systems ("Boost On"). Driven by resentment over his condition and a deep desire for a normal life, he formed a Binding Vow with Mahito and Kenjaku (Pseudo-Geto). He acted as a mole, leaking jujutsu society intelligence in exchange for Mahito healing his body using Idle Transfiguration. A vow stipulation forbade harming Kyoto Jujutsu High students or staff. Once healed, bearing only a single cheek scar, he immediately betrayed them, attacking to eliminate his collaborators, motivated by protecting his peers and severing enemy ties. For this final battle, he piloted his ultimate creation, "Ultimate Mechamaru - Mode: Absolute," a colossal tokusatsu-style mecha. Piloted from its head cockpit and powered by his entire lifetime's accumulated cursed energy (seventeen years, five months, and six days' worth), it briefly matched Special Grade strength and output. Its arsenal included the "Miracle Cannon" (a two-handed blast consuming two years' energy), "Pigeon Viola" (homing energy missiles consuming five years' energy), and the ability to deploy a New Shadow Style: Simple Domain barrier. He strategically used the Simple Domain to counter Mahito's soul manipulation, nearly defeating him. However, Mahito exploited Kokichi's momentary belief in victory, using Domain Expansion to counterattack and kill him inside the cockpit before he could reunite with classmates. Despite physical death, a fragment of his consciousness and residual cursed energy persisted within three pre-prepared "Mini-Mechamaru" units. During the Shibuya Incident, these small puppets aided Yuji Itadori and allies by warning of Satoru Gojo's sealing and providing tactical support against Kenjaku's forces until their energy depleted, marking his final act. His personality involved bitterness and jealousy born from confinement, notably resenting the free Panda. Yet he deeply cared for Kyoto classmates, especially Kasumi Miwa, to whom he confessed romantic feelings. His final posthumous message wished for Miwa's happiness. His tactical intellect was highly regarded, enabling complex strategies and opponent analysis that earned praise even from adversaries like Kenjaku.