Hopper Sensei, known simply as Hopper in the series Rilu Rilu Fairilu ~Yōsei no Door~, is a character whose role and appearance shift across the different seasons of the anime. He is a Fairilu, which are small magical beings born from seeds who live in the fairy world of Little Fairilu. His design is inspired by his motif as a grasshopper, and his name directly reflects this insect theme. His coloration primarily features a pale yellow-green scheme, aligning with his grasshopper identity. In the first two seasons, Hopper holds a distinguished status as a Legend Fairilu. A Legend Fairilu is a rare and powerful type of fairy, setting him apart from the more common flower, bug, or mermaid varieties. In this capacity, he serves as a coach for a sport called Shaborilu and also takes on the responsibilities of a physical education teacher, indicating that he is knowledgeable, active, and respected within the fairy community. He is also noted to be an active soccer player, further emphasizing his athletic and energetic personality. During these early seasons, another fairy named Jasmine, a Jasmine Fairilu, develops romantic feelings for him upon their first meeting, suggesting that he is a figure who commands attention and affection.

A significant change occurs in the third season, which is titled Oshiete Mahou no Pendulum: Rilu Rilu Fairilu and is set in a parallel universe. In this new continuity, Hopper appears in a child-like form, a stark contrast to his more mature teacher persona from before. His classification also changes; he is re-categorized as a Bug Fairilu, stepping away from his previous Legend Fairilu status. This version of Hopper sports piercings and his wings are displayed consistently with those of other Fairilu. As a child, he joins the main friend group centered around the protagonist Lip and her companions, allowing him to interact directly with both Fairilu and the human children who can access the fairy world using a device called the Magical Pendulum.

Given his role as a coach and physical education teacher, Hopper Sensei is inherently portrayed as a mentor figure. His motivations are tied to instructing younger Fairilu, helping them develop their skills in sports and physical activities, and fostering their growth. His background as a Legend Fairilu provides him with the authority and experience necessary for this teaching role. His development across the series is marked by this dramatic transformation from an adult authority figure and legendary being to a child-like peer of the main characters in an alternate reality. His notable abilities are primarily physical and athletic, centered on his proficiency in soccer and the fictional sport of Shaborilu, as well as his general fitness and coaching knowledge. While he is not one of the central protagonists like Lip or her closest friends, his role as a teacher establishes him as a key supporting character within the world of the first two seasons, contributing to the school setting and the community of Little Fairilu.