Asuka Toyama shares matching hair and eye colors with her energetic older sister, Kasumi, but possesses a calmer, more mature personality. Kasumi's cheerful and occasionally messy habits sometimes annoy the more responsible Asuka, though mutual care defines their relationship, shown through affectionate interactions like Kasumi kissing her cheek, embracing her from behind, and sharing a lap pillow. Kasumi affectionately calls her "A-chan."

During middle school at Hanasakigawa Girls' School, Asuka participated in the swim club, becoming acquainted with Yuri Ushigome. Upon graduating, she deliberately chose Haneoka Girls' High School over Hanasakigawa's high school section to prioritize its stronger academic focus for college preparation. At Haneoka, she became classmates and close friends with Rokka Asahi and Ako Udagawa. Her bond with Rokka is particularly close; she affectionately calls her "my Rokka," actively spends time with her, attends live music shows, listens to music together, and holds hands.

Though Rokka invited her to join a band, Asuka declined to concentrate on her studies. She remained a supportive presence, discreetly watching over Rokka during Rokka's search for compatible band members. After Rokka successfully formed her band, Asuka transitioned to volunteering as a staff member for the BanG Dream! Girls Band Challenge! event. Her development includes moments like playfully debating Ako about their respective older sisters' merits and defending Kasumi's qualities.

Asuka is distinguished as the only non-main character to receive a full 3D model in the anime's second and third seasons. She appears in the mobile game *BanG Dream! Girls Band Party!* with Live 2D animations, voiced lines, and featured illustrations on character cards.