Gilliam Jaeger hails from an unknown dimension, his ability to traverse parallel universes stemming from past actions that condemned him to perpetual displacement. Discovered unconscious and amnesiac on Planet Elpis's military training grounds, he enlisted in ALPHA Force as a Gespenst mecha pilot. Transferring later to the anti-terrorism unit ZEUS, he fought alongside heroes including Amuro Ray and Dan Moroboshi. His memories erupted during a mission rescuing a girl from a collapsing building, restoring his latent precognitive abilities and true identity.

Foreseeing Elpis's destruction, Gilliam adopted the alias "President Apollon" and launched Project Olympus. He unified criminal factions under Neo Axis, conquering cities from the dimensional fortress Helios to forge a united front against the apocalypse. Piloting the dimension-warping XN Geist, he clashed with former ZEUS allies. Upon defeat, he conceded their potential to reshape fate and let a collapsing dimension consume him during Helios's destruction. An anonymous farewell letter later confirmed his survival.

Banished to another universe, Gilliam continued interdimensional travel as "Helios Olympus," collaborating with Shadow Mirror and Tesla Leicht Institute to develop warp devices Lykeios and Aguieus, aiming to prevent System XN's abuse. An Aguieus test stranded him in the Original Generation universe, where he reclaimed his name and joined the Earth Federation Army's Aggressors unit. He battled alongside warriors like Sanger Zonvolt against Divine Crusaders and Aerogaters, all while evading Shadow Mirror's pursuit under Vindel Mauser.

Gilliam maintains a reserved, secretive demeanor, speaking minimally but unleashing intense anger when provoked. He hides his origins and potent precognition—exceeding psychics like Shine Hausen—forbidding its use after traumatic visions of apocalypse. Haunted by guilt over enabling Shadow Mirror's invasion of his new universe, he obsessively sought to return to Elpis. This culminated in a final clash against Vindel aboard the White Star, where Gilliam interfaced Aguieus to transport allies and foes to the Einst dimension. After defeating the Einst master, he activated System XN one last time with the code "ZEUS" to send everyone home. The system's destruction shattered his hope of returning to Elpis, compelling him to accept his current universe as home while watching for residual threats.

His primary mecha is the Gespenst, later upgraded to the high-mobility Type-RV, while the XN Geist served as his command unit during Project Olympus.