Akiha Shishidō, the third daughter of the aristocratic Shishidō family in a spacefaring society, possesses a careless and oblivious demeanor but inherent kindness, a laid-back attitude, and underlying courage. Her notable skill lies in piloting combat units known as QT-ARMS. Fleeing an arranged marriage orchestrated by her eldest sister, Kazane, she encounters the massive, sentient space colony Leopard, becoming central to subsequent conflicts.

Her journey intertwines with Leopard, a malevolent and eccentric artificial intelligence. Their relationship evolves from friction to mutual dependence. Akiha wields the golden firearm Arma Ignis Aureus, which interfaces with Leopard's systems to unleash the powerful "Soul Shout" energy attack from the Leopard Cannon. This synergy proves crucial in battles, including destroying a rogue space plant and confronting adversaries like Benkei. However, Akiha exhibits susceptibility to mental manipulation by Nervalist forces promoting subjugation under the AI Nerval. Confined within a specialized containment unit (Full Cowl box) aboard colony Xanthippe, she resists Nerval's alluring control with support from her robotic companion, Imoko "Imo-chan" Shishido.

Akiha confronts her perceived inadequacies compared to her accomplished sisters and embraces her inherited legacy. She demonstrates resilience during psychological attacks, aided by interventions from Honoka Kawai and Itsuki Kannagi. The entity EXISTENCE protects her during an assassination attempt by her sister Nami. Akiha inherits a powerful "will" from Kagura, a figure linked to past conflicts against Nerval, solidifying her resolve. Kazane later urges her to destroy Leopard, adding familial pressure.

Primarily piloting the war-era Starsylph QT-ARMS, Akiha transitions from reluctance during missions to retrieve components for Leopard to proactive engagement. Her piloting proves instrumental in critical events, preventing Leopard's self-destructive colony drop on Earth and fighting large-scale battles against Nervalist forces and hostile colonies. Her growth culminates in directly confronting Leopard's "Prince of Darkness" persona, freeing him from possession through decisive action.

Her foundational relationship with Imoko, adopted as the sixth Shishidō sister, provides unwavering loyalty, technical expertise, and emotional stability, vital during doubt and external threats. Her interactions with Leopard deepen into interdependence, highlighted by his distress when she is endangered and his reliance on her to activate his most potent weaponry.