Junichi Tajima serves in the 108th Guard Squad, stationed in Aomori. This unit suffers from poor equipment and low standing, widely considered a "reject camp" for pilots who failed to qualify for Hokkaido's elite forces. Tajima himself was demoted and transferred to the 108th after an insubordination incident ended his service in an elite combat unit. This past makes him one of the squad's veterans.

Tajima holds advanced technical knowledge in mecha maintenance and combat tactics. He applies this expertise to support the unit despite its outdated gear. He maintains a gruff, cynical demeanor, particularly toward authority and the military hierarchy, reflecting his disillusionment post-demotion. Yet, he displays protective instincts toward younger or less experienced members, notably mentoring Shinobu Kurano. His technical skills prove vital when the squad encounters equipment failures or tough combat situations.

During defensive operations, Tajima engages in frontline combat, leveraging his piloting skills. He participated in the Battle of Lake Towada, documented providing covering fire against Phantom Beasts for squadmates. His combat actions reveal a growing sense of responsibility for his unit, marking a shift from initial detachment to active commitment in protecting fellow pilots. This underscores his transition from focusing on personal grievances to recognizing his role within the 108th Guard Squad.