Hina is a main female character and a Guardian Heart, a being tasked with protecting the peace and well-being of an assigned planet, with her mission being Earth. Her background is defined by failure; she is considered a dropout due to previous missions where she failed to keep her identity a secret, a record she is said to hold. Her assignment to Earth serves as her final chance for redemption, under the strict condition of absolute secrecy. This mission is compromised almost instantly when a high school boy named Kazuya Watari witnesses her arrival transformation, setting a new record for the fastest exposure of a Guardian Heart. To salvage her mission, she invokes protocols that exempt family members from the secrecy rule, and consequently moves into Kazuya's home under the pretense of being family, a decision his mother approves over his objections. To remain on Earth, she must also periodically pass evaluations given by her superiors.

In terms of personality, Hina is defined by her growing romantic feelings for Kazuya, which manifest as strong possessiveness and jealousy. She actively interferes when other girls approach him, often using her Guardian Heart abilities to physically intervene. This behavior puts her in direct conflict with the other female characters who come to live with Kazuya. Her motivations are twofold: to fulfill her duties as a planetary protector to secure her redemption, and to protect her personal relationship with Kazuya from rivals. The interpersonal dynamics of the household constantly complicate her official duties.

Her key relationships are primarily with the other girls in Kazuya's household, all of whom are unaware that Hina is the Guardian Heart. She battles the runaway ninja Maya when Maya attempts to kill Kazuya for discovering her identity; this fight leads to Kazuya helping Maya escape, creating a life debt. She also interrupts Chelsea's accidental magical girl reveal and Kurusu's alien confession to Kazuya, generating resentment from both. Chelsea, a princess from a magical world who believes Kazuya is fated to be her husband, hates Hina for always getting in her way without realizing Hina is the Guardian Heart she also despises. Maya similarly hates the Guardian Heart for defeating her in battle. In the sequel series, Guardian Hearts Power Up, the rivalry intensifies with the arrival of a woman claiming to be Kazuya's future fiancée, while a new antagonist forces the girls into conflict.

Throughout the story, Hina shows development by continuing to balance her guardian duties with the comedic and confrontational interactions driven by her fixation on Kazuya and her competition with the other girls. Her role in the story is that of the catalyst; her accidental reveal sets the premise for the entire harem comedy, as every subsequent girl whose secret Kazuya discovers also comes to live with them. Her notable abilities include those of a Guardian Heart, which grant her the power to protect and physically intervene in conflicts, as demonstrated when she fights Maya. She also possesses a transformation sequence related to her arrival on Earth.