Maasa Shirahone, originally named Magozaemon Shirahone, endured childhood bullying fueled by morbid obesity and an old-fashioned name. His cousin, Ichiro Dougo, attended the same elementary school yet misread their classmates' cruelty. Ichiro unintentionally encouraged Maasa's overeating and cast him in a school play role emphasizing his weight, deepening Maasa's resentment. After moving away for roughly four years, Maasa changed his name, adopted a strict no-carbohydrate diet, and embraced a healthier lifestyle, achieving substantial weight loss upon enrolling at Binan High School. Despite Ichiro also attending Binan High, Maasa deliberately avoided interaction, angered by Ichiro's past obliviousness.

As a first-year student, Maasa secured the student council secretary position, seeking belonging within a group he perceived as stylish and cool. He wears an inverted Binan High uniform: a white blazer over a black shirt, white suspenders, and brown shoes, distinct from other council members. His personality blends superficial charm with underlying vindictiveness; he cheerfully distributes handmade sweets declaring "Everyone~ Eat it~!", but intentionally laces them with fattening, addictive ingredients to induce weight gain in others—a revenge tactic rooted in his childhood trauma. He intensely values beauty, dismissing anything or anyone deemed uncool, uncute, or unbeautiful, and reacts strongly when his own appearance is compromised.

Aligned with the student council's conquest agenda under Prince Furanui, Maasa transforms into the Magic Knight Ritter Amethyst using a ring called Hart, wielding amethyst-based powers. His knight form features an Arabic-style blue and black suit with gold accents, a violet chest plate, and a cape. Though loyal to the Edelstein group and compliant with their plans for their perceived coolness, he occasionally voices frustration with Furanui, labeling him "uncute". Maasa frequently targets Ichiro during conflicts, leveraging their shared history to provoke him.

Throughout his arc, Maasa demonstrates gradual self-acceptance, declaring "Maasa is Maasa" to affirm his identity separate from his past as Magozaemon. Nevertheless, reminders of his former self, especially when Ichiro uses his birth name, trigger intense anger, highlighting unresolved trauma. His hobbies include making sweets and occasional use of English phrases, though his primary focus remains maintaining his self-image and group status.