Kiel Aronnax is a scientist with formidable intellect and a physically imposing warrior's physique. Falsely imprisoned at Alcatraz, he arranged his own incarceration to investigate the facility's hidden secret. He survived torture and harsh conditions under the corrupt Warden Kihano, steadfastly pursuing the prison's mysteries.

His imprisonment overlapped with a breakout attempt by treasure hunters Loretta Oratorio and Mirabelle Graceland, seeking rumored riches within Alcatraz. Kiel allied with them, offering guidance to the secure vault in exchange for escape assistance. Inside, they discovered the "treasure" was Sheyenne Rainstorm, a comatose adolescent boy. Kiel recognized the name and took immediate interest, especially upon learning Sheyenne could wield rare ARMS technology and claimed his adult consciousness was trapped in a child's body after a brain transplant.

Kiel masterminded the group's escape from Alcatraz, using strategic thinking and physical prowess to overcome guards and a monstrous Falconvein. He demonstrated atypical combat skills and proficiency in hacking security systems. After escaping, Kiel chose to accompany Sheyenne on the quest to find his original body, motivated by scientific curiosity about the transplantation and a deeper, undisclosed connection to Sheyenne's past.

Throughout the journey, Kiel acts as a moral anchor and reluctant father figure, particularly to Sheyenne. The boy's juvenile appearance often leads outsiders to mistake them for father and son—a dynamic Sheyenne resents. Despite his muscular build and capacity for violence, Kiel exhibits humility, preferring his first name over the formal title "Doctor." His calm, thoughtful demeanor provides stability, though he harbors significant secrets concerning Sheyenne's condition and the origins of ARMS weaponry. These secrets form a crucial yet concealed aspect of his involvement in the mission.