Xavier Foster, known as Kiyama Hiroto in Japanese media, initially presents as an ordinary observer fascinated by Mark Evans and Raimon's soccer matches against Alius Academy, concealing his deeper ties to the antagonistic forces.

He resurfaces under the alias "Xene" (Gran in Japanese) as captain of Genesis, Alius Academy's strongest team. Leading Genesis against Raimon in Fukuoka, his team dominates until a teammate exposes his Xavier Foster identity, fracturing his bond with Mark. Visibly anguished by the severed friendship, Xavier battles conflicting loyalties. Raimon's eventual victory catalyzes his realization of manipulation by his Alius Academy father figure, prompting him to reject that legacy and reconcile with Mark and Raimon.

Post-Genesis, Xavier transitions to a protagonist role. He joins Inazuma Japan for the Football Frontier International tournament as a pivotal attacker. He collaborates with Axel Blaze on the "Grand Fire" technique, scores decisive goals like one against Little Gigantes, and temporarily captains the team during Mark's absence. His cooperation extends to moments like aiding Scotty when lost in a forest.

His techniques evolve: "Meteor Blade" and "Super Nova" with Genesis, expanding to "The Dawn" and "Celestial Smash" with Inazuma Japan while retaining "Meteor Blade." Game data lists him as a Fire-element Forward with 111 FP, 129 TP, and max-level attributes emphasizing Kick (71), Speed (69), and Control (67), reflecting his technical attacking prowess.