Maya Ōba, alias "Black Maya," hails from a secretive ninja clan. She fled her clan for undisclosed reasons yet still adheres strictly to its traditions, including the mandate to eliminate anyone discovering her ninja identity. This forces her to attempt Kazuya Watari's elimination after he accidentally uncovers her ninja tools. Guardian Heart Hina's intervention sparks a confrontation, during which Kazuya aids Maya's escape. Following clan custom, Maya incurs a life debt to Kazuya and moves into his home to fulfill this obligation.

Further clan traditions dictate that Maya must marry the first man who sees her naked, which occurs twice with Kazuya, compounded by two unintended kisses. Initially resentful due to these obligations and her defeat by Hina, Maya harbors animosity towards both Kazuya and the Guardian Heart. Over time, she comes to recognize Kazuya's reliability and kindness, accepting him as her fated husband. Despite this internal shift, she struggles to articulate her feelings outwardly.

In the sequel OVA *Guardian Hearts! Power Up!*, Maya continues to reside in Kazuya’s household. Rivalry intensifies among the magical girls vying for his affection, escalating with the arrival of new characters, including a woman claiming to be Kazuya’s future wife. Maya actively participates in the resulting competitions and confrontations, reflecting her persistent commitment to Kazuya and ongoing friction with Hina.

Her character arc transitions from hostility and rigid adherence to tradition towards reluctant affection and loyalty to Kazuya. Her ninja identity and rivalry with the Guardian Heart remain consistent elements throughout her interactions.