Hajime Saitō served as a non-commissioned officer in the United Nations Cosmo Marines, bearing a distinctive star-shaped scar on his left cheek from a military injury. He maintained a close friendship with Colonel Goro Kiryu and his family, with Kiryu's daughter Mikage viewing him as an older brother figure.

In early 2199, Saitō was stationed with the 7th Regiment on Earth's moon when Garmillas forces devastated their base, causing heavy casualties including Colonel Kiryu's death. As the highest-ranking survivor, Saitō confronted Admiral Ryu Hijikata aboard the evacuation ship Kirishima over the delayed rescue and lost comrades. This outburst prompted Hijikata to disclose the existence of the Yamato as Earth's final hope.

By December 2199, Saitō transferred to riot control in Earth's Far-East District underground city. Tasked with suppressing civil unrest amid worsening conditions, his frustration at firing upon fellow humans drove him to breach the UN Space Planning Office command center. He confronted Chief Executive Heikuro Todo and Admiral Hijikata directly, with Hijikata offering reassurances before news interrupted them of the Yamato's return.

By 2202, Saitō commanded the UNCM 5th Regiment on the outer solar system's Eleventh Planet. During a Christmas celebration, his post faced ambush by the Gatlantis Empire's vanguard fleet. With communications severed and a blockade imposed, he ordered survivors to evacuate while directing Master Sergeant Shiori Nagakura to breach enemy lines for aid. This action triggered the Yamato's intervention and their eventual rescue.

Saitō exhibited fierce loyalty to the Cosmo Marines and United Nations Cosmo Military values, prioritizing these principles over protocol when compromised. His forthright attitude challenged authority yet earned quiet respect from figures like Hijikata, while his relationship with Mikage Kiryu revealed a playful, protective dimension.