Asuka Toyama, younger sister of Kasumi Toyama, initially attended Hanasakigawa Girls' School as a third-year junior high student and swam in the club with Yuri Ushigome. Prioritizing academics over joining her sister at Hanasakigawa's high school, she enrolled in the rigorous Haneoka Girls' High School. There, she became classmates and close friends with Rokka Asahi and Ako Udagawa.

Her calm, responsible, and academically focused personality starkly contrasts Kasumi's energetic impulsiveness. Though sarcastic and reserved in showing affection, Asuka shares a complex yet caring bond with her sister, demonstrated through herb gardening, lap pillows, and cheek kisses. Their relationship evolved from elementary school clashes to a friendship-like connection.

During her first Haneoka year, Asuka declined Rokka's band invitation to concentrate on studies but voluntarily supported her friend's search for "fated band members," intervening when outsiders like Chiyu Tamade approached Rokka at school. After Rokka formed RAISE A SUILEN, Asuka served as staff for the "BanG Dream! Girls Band Challenge!" at Budokan, observing performances by Poppin'Party, Roselia, and RAISE A SUILEN.

Subtle displays of protectiveness toward Rokka emerged, including holding hands and sharing music-listening moments in season three's ending sequence. Her stoic exterior also cracked during a lighthearted debate with Ako about their sisters, revealing hidden admiration for Kasumi. As the only non-main character with a full 3D model in the anime's second and third seasons, Asuka maintains a recurring narrative role.