Julio Santana, once the world's fourth-ranked Bakugan brawler, wielded the Haos attribute from his European base alongside his Guardian Bakugan, Haos Tentaclear. A fierce rivalry with fellow Haos expert Runo Misaki led her to dub him "Cue-Ball." Masquerade's trap captured Julio, transporting him to the Silent Core where Naga imposed mental control. Forced to battle Runo and Dan Kuso under this influence, he doomed four of their Bakugan before losing. Later, allied with possessed brawlers Chan Lee and Klaus von Hertzon, he confronted Dan, Runo, and Marucho Marukura, only to be defeated and freed from Naga's grasp. Liberated, Julio surfaced during Naga's Earth invasion, stalling the hybrid Tricloid for the resistance. He then joined Komba O'Charlie and Shun Kazami to defeat Hairadee. In the final clash against Centorrior and Druman, he fought alongside Dan, Runo, Julie Makimoto, Klaus, Billy Gilbert, and Komba, securing victory. After Naga's defeat, he tearfully parted with Tentaclear as all Guardian Bakugan returned to Vestroia. Julio reappeared non-combatantly in *New Vestroia* via a flashback group photo. Narrative implication suggests he reunited with Tentaclear after the Resistance freed Bakugan from Vexos imprisonment. His battle record includes wins and losses, notably an off-screen defeat by Masquerade that doomed Tentaclear and two other Bakugan. His appearance features a bald head and black eyes.