Koichi Nanase, alias KO-1 (a phonetic play on his name), was once an elite Gunpla builder and pilot. His competitive career featured notable successes: third place in the Japanese Gunpla Championship and eighth in the World Tournament, signaling potential to dominate globally. This trajectory halted abruptly when his team disbanded during the transition from the physical Gunpla Duel (GPD) system to the virtual Gunpla Battle Nexus Online (GBN), forcing his retirement from Gunpla Battle for four years.

During this hiatus, he constructed rental Gunpla units at The Gundam Base Tokyo. His younger sister, Nanami Nanase, also employed there, actively participated in GBN while Koichi remained detached from its community. His isolation ended upon meeting Riku Mikami and friends, whose fervor for restoring his old Gunpla and dedication to the game reignited his competitive drive. Koichi returned to battle, joining their Build Divers team as a veteran pilot of his custom RMS-117G11 Galbaldy Rebake.

This comeback transformed him from a disillusioned former champion into a mentor guiding newer Divers. The Galbaldy Rebake—crafted as an homage to the ASW-G-11 Gundam Gusion Rebake from *Mobile Suit Gundam IRON-BLOODED ORPHANS*—remains his primary unit.