Ema Asahina, originally introduced as Ema Hinata, is the central character of the story. She is a sixteen-year-old high school student at the start of the narrative, later advancing to university. Her background is marked by loss and a degree of solitude. Orphaned at a young age after the death of her biological parents, she was adopted and raised by Rintarou Hinata, an adventurer whose work often took him away from home for long stretches. This upbringing instilled in her a sense of independence, as she became accustomed to fending for herself and spending many days alone. A significant turning point in her life occurs when her father remarries Miwa Asahina, leading Ema to move into the Asahina household and gain thirteen new stepbrothers, changing her surname to Asahina in the process.

In terms of personality, Ema is consistently portrayed as a kind-hearted, polite, and honest individual. She is a dutiful daughter who cares deeply for her new, extended family and strives to be a positive presence in their lives. Despite the sometimes overwhelming attention from her many brothers, she generally remains composed and tries to navigate her new situation with grace. She is not portrayed as overly assertive but rather has a gentle and approachable demeanor. Her capability in the kitchen is a notable trait, as she is skilled at cooking and particularly adept at making desserts. Beyond her domestic talents, Ema has unexpected hidden depths; she is interested in video games and anime, enjoying zombie-themed games and the titles produced by the company where one of her stepbrothers, Natsume, works.

Ema's primary motivation is to find a sense of belonging and familial connection, something she lacked growing up due to her adoptive father's frequent absences. Her role in the story is that of a catalyst and a focal point. Her arrival in the Asahina household disrupts the existing dynamic among the brothers, all of whom develop romantic feelings for her. She serves as a bridge between the siblings, and the narrative revolves around her interactions with them and her gradual adjustment to her new life. Her relationships with each brother are distinct. She acts as a caring older sister figure to the youngest brother, Wataru. She shares a bond with the twins Tsubaki and Azusa, and she bonds with Natsume over a shared interest in video games, even providing him with feedback on his work. Her classmate and stepbrother, Yuusuke, had a crush on her even before she joined the family, creating initial tension.

Throughout the story, Ema undergoes significant development. A key moment is the revelation that Rintarou Hinata is not her biological father. This discovery shocks her deeply, leading to an identity crisis where she feels like an unwanted child. However, with the support of her stepbrothers, particularly Natsume and Louis, and her pet squirrel Juli, she comes to terms with the fact that she is truly loved as a daughter, regardless of blood ties. This event strengthens her resilience. Her romantic development culminates in a choice; in the source novels, she ultimately selects and enters a relationship with Subaru, one of her younger stepbrothers, with whom she later starts a family. In the anime adaptation, however, she does not make a definitive romantic choice among the brothers.

One of Ema's most notable and unique abilities is her capacity to understand her pet squirrel, Juli, who can speak and is fiercely protective of her. Only she and her stepbrother Louis are able to comprehend Juli's words. This ability makes Juli her constant companion and confidant, providing her with advice and protection in her new, crowded home.