Ikkū Ameya is a member of the immortal organization UQ Holder, designated as Number 10. His existence is a product of both tragedy and advanced science, as he is a full-body cyborg whose immortality was achieved through a process known as cyberization. Born in the year 2001, Ameya’s life took a drastic turn at the age of thirteen when an accident left him in a comatose state. He remained bedridden for seventy-two years, making his legal age eighty-five, while his consciousness remained that of a thirteen-year-old. To grant him a new life, he was given a synthetic, mechanized body, and he chose to present himself as being twenty-four years old. He joined UQ Holder as a gesture of gratitude for their help and with the resolve to protect the world he cares for.

In terms of personality, Ameya is characterized by a relentless and warm-hearted optimism. He is almost always seen with a smile on his face and often encourages others, even stern individuals like his colleague Karin, to do the same. His kindness is genuine and spontaneous, demonstrated when he calms distressed children and repairs a broken toy for them without hesitation. As a relatively newer member of the organization when the main protagonists join, he takes on a welcoming and explanatory role, giving the newcomers Touta Konoe and Kuroumaru Tokisaka a warm reception and patiently explaining the group's purpose as a sanctuary for non-humans. His motivations are deeply rooted in his second chance at life; he serves UQ Holder to defend the world he loves and shows a playful yet determined spirit in his duties.

Within the story, Ameya acts as a supportive ally and a source of both comic relief and formidable firepower. He is present for the initiation test of Touta and Kuroumaru and is one of the first to welcome them into the fold. He participates in several key missions, providing crucial support. He assists in destroying anti-mage tanks, joins the group to confront the powerful mage Fate Averruncus, and uses his unique abilities to break through magical barriers. He also infiltrates the Amano-Mihashira Academy City using a younger, thirteen-year-old body model to blend in with other characters. His development takes a significant and tragic turn in a future storyline set forty-five years later, where he becomes a formidable enemy. After uploading his personality to the internet, his mind is eroded and taken over, forcing his former comrades to fight him. In a final confrontation, he is brought back to his senses by a comrade's ability and, expressing his gratitude, fades away as his life support fails, returning to his original, aged body one last time.

Ameya’s notable abilities are almost entirely derived from his cybernetic nature. His mechanical body grants him enhanced strength, speed, and the ability to fly using built-in rocket boots and jet packs. He is a living arsenal, equipped with various weapons such as arm-mounted lasers, missiles, and powerful combat limbs. One of his most powerful assets is the Al-Iskandariya, an orbital cannon he can call upon to fire a devastating laser from space, strong enough to break through high-level magical defenses. This weapon is a Pactio artifact, an item granted through a magical contract, which he inherited from a character from the related series Negima! Magister Negi Magi. Ameya also forms his own Pactio contract with Touta Konoe, which grants him a full-body powered exoskeleton that can be upgraded, significantly enhancing his combat capabilities. True to his scientific origins, he continuously upgrades himself, and over forty-five years he utilizes a vast amount of computing resources to modify his body into a state powerful enough to contend with the organization's strongest fighters. His immortality is specifically reliant on his ability to transfer his consciousness between different synthetic bodies, making him exceedingly difficult to permanently destroy as long as he has a functional backup.