Eriko Nanao is a prominent character from the anime, introduced as a nineteen-year-old college student and the second cousin of the protagonist, Ryuuji Kisaragi. Having spent the previous three years in the United States, she returns unexpectedly to Japan at the start of the story, bringing with her a bold and driven personality that immediately sets events into motion. Physically, she is depicted as a young woman with long brown hair and a confident, mature demeanor that often contrasts with the more reserved nature of those around her.

As a Breaker, an individual capable of using mystical artifacts known as Lost Precious, Eriko holds a level seven ranking, though some sources indicate her power measures closer to level eight under certain conditions. Her personal arsenal includes several notable Lost Precious items. She wields a staff, a B-class artifact that creates illusory swarms of butterflies to confuse and disorient opponents. She also possesses a pair of earrings called True Love, which are A-class Lost Precious that grant the wearer the ability to hear only the truth from whomever they are speaking to. These earrings are a family heirloom that the Nanao clan has long sought.

Eriko's personality is defined by her overwhelming self-confidence, spontaneity, and energetic nature. She is a woman of action who prefers to act first and think later, often dragging Ryuuji into dangerous situations without much explanation. Her preferred method of transport involves a red BMW sports coupe, which she drives with a reckless yet highly skilled abandon that mirrors her approach to life. Despite her sometimes overbearing and forceful manner, she is deeply caring and acts as a de facto guardian for Ryuuji, moving into his home rather than staying in her own empty apartment. She has a lively and open-hearted disposition, making no secret of her emotions and clearly enjoying being surrounded by people she cares about.

Initially, her primary motivation is to gain recognition from the Society, the governing body for Breakers, by successfully acquiring a powerful S-class Lost Precious being sold by a criminal organization called Fang. To this end, she forcibly recruits Ryuuji as the first member of her self-styled organization, Seven Tails. However, when the mission results in the discovery of a red dragon girl rather than a simple artifact, Eriko's focus shifts. The dragon, later named Rose, becomes an integral part of their household, and despite Rose's initial hostility toward her, Eriko works to support and protect her, eventually developing a sisterly bond with the young dragon.

Throughout the story, Eriko serves as a key supporter and ally to Ryuuji. She provides transportation, combat assistance, and comic relief while also acting as a bridge between the normal world and the secret world of Breakers and Lost Precious. Her relationship with Ryuuji is close and affectionate; she is unafraid to show him physical affection and clearly treasures their familial bond, having missed him during her time abroad. While she can be manipulative to get what she wants, her intentions generally stem from a desire to help and a love for adventure. Her character shows development from a lone operative seeking external validation to an integral member of an unconventional family, finding satisfaction in her relationships with Ryuuji and Rose rather than solely in the approval of the Society.