Hina, a Guardian Heart classified as a dropout after previous mission failures, is assigned to Earth as her final chance for redemption, contingent on absolute secrecy about her identity. This mission is instantly compromised when Kazuya witnesses her arrival transformation, setting an unprecedented record for the fastest exposure of a Guardian Heart. To preserve her mission under protocols exempting family, Hina moves in with Kazuya. She must also pass periodic evaluations from her superiors to remain on Earth. While her initial role focuses on planetary protection, interpersonal dynamics complicate her duties.

Hina develops romantic feelings for Kazuya, leading to possessive behavior. She actively interferes when other girls approach him, often using her Guardian Heart abilities to physically intervene. This jealousy sparks conflicts: she battles ninja Maya when Maya tries to eliminate Kazuya for discovering *her* identity, leading Kazuya to aid Maya's escape and Maya owing him a life debt. Hina also interrupts Chelsea's accidental magical girl reveal and Kurusu's alien confession to Kazuya, causing both girls to resent her while remaining unaware of Hina's true identity.

In the sequel, the arrival of a woman claiming to be Kazuya's future fiancée intensifies rivalry among the girls. Simultaneously, a new antagonist forces the girls into conflict while draining their "innocence." Hina continues balancing her guardian duties with comedic and confrontational interactions driven by her fixation on Kazuya and competition with the other girls.