Himeko Mashima, a striped cat Myumon born on August 22nd and standing 158cm tall, energizes the band Mashumairesh!! as lead guitarist and vocalist, wielding her signature instrument "Mashima Twinkle." Her vivid style includes long blond-and-pink twisted twintails, magenta eyes, and fair skin highlighted by pink streaks across her bangs. She completes her look with a maroon ribbon bow, a yellow-sparkled pink coat layered over a bubblegum pink Lolita dress, coordinated stockings, and maroon high-heel ankle boots.

Her strong-willed and occasionally cynical nature contrasts with her fierce dedication to musical improvement. A compelling dynamic unfolds with bandmate Howan, marked by evolving chemistry and mutual growth throughout their journey. This relationship deepens as Himeko undergoes personal development, embracing introspection and sharpening her commitment to the band's ambitions.

Himeko's enthusiasm ignites when encountering influential bands like Criticrista, where she secures backstage access for Mashumairesh!! to study their craft. Observing Rosia's professionalism amid adversity sparks her recognition of her own band's limitations, driving rigorous training. Her competitive spirit flares during clashes with rivals such as the all-girl trio Reijingsignal.