Ema, originally Ema Nagami, becomes the adopted daughter of adventurer Rintarou Hinata after his remarriage to Miwa Asahina, adopting the Asahina surname and gaining thirteen stepbrothers. She has light brown hair often in a side ponytail, brown eyes, fair skin, and a petite, slender frame usually hidden by loose clothing. Her typical attire is a school uniform featuring a sailor top, cream cardigan, black pleated skirt with a pink stripe, knee socks, and loafers, switching to casual wear outside school.

Kind-hearted, polite, and soft-spoken, Ema flusters easily at her brothers' romantic overtures. Despite her gentle demeanor, she holds firm convictions, rejecting dishonest relationships and consistently communicating her desire for purely familial bonds with her stepbrothers, though they often misinterpret her kindness. Her past as an only child with an absent father fuels a deep longing for family closeness, resulting in strong devotion to Rintarou and Miwa. Witnessing escalating conflicts among her brothers over romantic rivalries causes her significant distress and guilt, making her feel like an outsider disrupting harmony. She gradually develops greater resilience, striving to uphold her boundaries while hoping for eventual family unity.

Key relationships include her affectionate bond with Rintarou, whom she calls "Papa," and her pet squirrel Juli, her confidant and protector against her brothers ("the wolves"). Juli communicates verbally with her and stepbrother Louis. Notable interactions with brothers include: Masaomi acting as her guardian during Rintarou's absences; Ukyo collaborating on household cooking; Kaname's persistent flirting, including a forehead kiss; Tsubaki's forceful advances, including non-consensual kissing; and Fuuto nearly assaulting her while she slept before stopping himself. Despite these complexities, Ema remains committed to establishing platonic sibling relationships.

Her development centers on confronting her adoption, initially triggering an identity crisis and withdrawal. Supported by Natsume, Louis, and Juli, she reconciles this truth, recognizing family transcends blood. Later arcs, including university settings, see her navigating academic pressures while managing her brothers' affections. Alternate portrayals diverge: the light novel depicts her more assertively rejecting advances, while game routes explore scenarios where she reciprocates feelings for individual brothers, leading to committed relationships—such as marrying Subaru and having a child in novels, or engaging in physical intimacy with characters like Kaname, Tsubaki, or Hikaru in game narratives.