Introduced in the first season, Subterra-attribute brawler Billy Gilbert is Julie Makimoto's childhood friend and a talented baseball player. Initially ranked tenth globally, he succumbs to brainwashing by Naga's Silent Core energy, becoming a henchman for Masquerade. In this state, he battles former allies Julie and Shun, nearly banishing Julie's partner Gorem to the Doom Dimension. Julie redeems him by revealing a childhood toy he gifted her, triggering his memories and breaking Naga's control. Restored to normal, he aids the Battle Brawlers against Naga, participating in group battles against Rabeeder, Tricloid, Centorrior, and Druman. His Subterra Cycloid is temporarily sent to the Doom Dimension but later rescued. After the first season, Billy and Julie begin a romantic relationship. During New Vestroia, he travels globally, learning from athletes in Spain and aiding school-building projects in Africa. He reunites with Julie for a tag-team match against Ace Grit but is defeated, inadvertently exposing the Resistance's hideout to the Vexos. He volunteers to disable the Bakugan Termination System but stays behind during the final assault on the Vestal Motherpalace. He does not reappear in subsequent conflicts beyond a brief mention in Gundalian Invaders, where Julie notes his absence during her travels. Billy sports freckles, long yellow hair matching Marucho's color, and clothing with lightning bolt motifs on his hat and jacket, often wearing fingerless gloves. His playful personality includes calling Julie "BakuBabe," irritating her. His Bakugan-throwing style mirrors a baseball pitch. Beyond Cycloid, he uses Subterra Bakugan like Wormquake, Hynoid, and Tuskor.