Born from pure Angolmois Energy interacting with Lio Convoy's Energon Matrix during the commander's recovery in an infused cave on Gaia, Lio Junior emerged as a distinct robotic lifeform. Initially manifesting as a lion cub, he was raised by the white lion that saved Lio Convoy, unaware of his origins. His encounter with the Maximals changed everything; Tasmania Kid deduced his connection to Lio Convoy, dubbing him "Lio Junior," a name the cub embraced. This sparked Lio Junior's conviction that Lio Convoy was his father, creating an intense emotional bond and desperate need for paternal recognition.

Lio Convoy initially rejected this familial claim, focused on commanding the war against Galvatron's Predacons. This fueled Lio Junior's feelings of inadequacy and a persistent drive to prove himself worthy to the Maximals, who often dismissed his youth. His impulsive actions, like charging into the Autorollers' base solo or attempting to absorb corrupted Angolmois Energy for power, frequently escalated danger. This recklessness made him vulnerable to manipulation. Galvatron exploited his guilt and longing for approval, tricking him into surrendering a critical teleport gate control unit, which summoned the destructive Majin Zarak.

Lio Junior's training and integration into the Maximal forces were guided by mentors Skywarp and Santon, who arrived shortly after his creation. Together, the trio could combine their bodies and minds into the powerful combiner Magnaboss, with Lio Junior at its core. This form became a symbol of hope and proved pivotal against Galvatron's upgraded forces, including the Cyborg Beasts. A turning point came during a near-fatal encounter with Gigastorm. Rescued by Lio Convoy from a collapsing cliff, Lio Junior received private acknowledgment as his son, Lio Convoy explaining his earlier distance stemmed from wartime necessity. This fostered mutual understanding, though Lio Junior subsequently addressed him as "commander" professionally.

Lio Junior wielded abilities derived from his lion beast mode and Angolmois Energy connection. His tail transformed into the "Lio Byūto" whip for combat, while twin horns extending from his mane fired concentrated Angolmois Energy spheres. His mane could also form temporary wings for short-distance flight in robot mode. In the final battle against Galvatron, who had absorbed colossal Angolmois Energy to monstrous size, Lio Junior combined directly with Lio Convoy. This fusion created a powerful green lion form capable of mortally wounding Galvatron and containing the resulting Angolmois explosion, saving Gaia. This act cemented Lio Junior's place as a respected Maximal warrior, validating his journey to maturity.