Asa Kato operates aboard the flying battleship Amanohara during the 1904 Russo-Japanese War as a member of the Lime Unit, a group of teenage girls wielding supernatural Konjiki powers. Her tomboyish, aggressive, and stubborn personality stems from a military upbringing under her father, shaping her commanding demeanor and kendo proficiency. Potentially the youngest unit member, she displays fierce competitiveness, particularly in clashes with Sarasa Honda, whose upper-class affectations contrast with Asa’s directness. She experiences A-Cup Angst as the flattest-chested member, drawing frequent teasing.

In combat, she pilots the green unmanned mecha Kiyomasa, wielding a staff and channeling plant-based "Green Thumb" abilities. Her backstory centers on a close bond with Lieutenant Commander Masanosuke Date, the unit’s original commander. She regularly practiced kendo with him and harbored deep affection, making his sacrificial death—shielding Sarasa from an enemy mecha—a pivotal trauma. This loss fuels her initial resistance toward the new civilian commander, Shintaro Umakai, though she gradually accepts his leadership.

She shares a strong friendship with timid unit member Kinu Fukushima, though her forceful reactions occasionally reduce Kinu to tears. Notably, Asa remains the sole Lime Unit member Shintaro never accidentally sees naked or in her underwear. During non-combat activities, like a tropical island festival in an OVA, she participates assertively in group efforts to attract Shintaro’s attention. Her design includes casual schoolgirl outfits, and she surprises others during a formal ball with a polished appearance.