Kayo Maruta is a supporting character in the Citrus series, easily recognized by her short bobbed black hair and large glasses, which have earned her the affectionate nickname Glasses Senpai from Harumi Taniguchi. She is a student at the prestigious Aihara Academy. Beginning as a second-year student, she serves as a dedicated member of the student council and holds the position of vice president. By the conclusion of the main storyline, she has graduated from the academy. Kayo typically keeps her glasses on at school, though she chooses to remove them when going outside.

In terms of personality, Kayo is portrayed as a conscientious and hardworking individual who takes her duties on the student council seriously, often aiming to uphold school rules and discipline. Despite this strict side, she maintains a friendly and approachable aura. Other sources characterize her as shy and meek, noting that she is prone to stuttering and can be easily flustered, which distinguishes her from some of her more assertive colleagues on the council. Beneath this reserved exterior, she possesses a sharp intellect and a keen sense of observation. Kayo displays notable empathy towards her peers, showing a particular ability to notice subtle changes in Harumi Taniguchi's mood and offering her support. Her approach to council duties, which involves balancing a respect for rules with compassion for fellow students, demonstrates her maturity. Outside of the school environment, she appears to adopt a more relaxed demeanor.

Kayo’s primary role within the story is as a supporting figure within the student council, where she works alongside the president, Mei Aihara, and the vice president, Himeko Momokino. Despite being an upperclassman, she treats the younger Mei and Himeko with respect for their positions. A significant part of her function involves monitoring students who challenge school regulations, which often brings her into contact with the free-spirited Yuzu Aihara. Kayo routinely reprimands Yuzu for violations of the dress code and other rules, marking one of her key interactions within the narrative.

Kayo’s key relationships provide considerable depth to her character. Her most notable connection is with Mitsuko Taniguchi, Harumi’s older sister. While many students find Mitsuko intimidating, Kayo communicates with her confidently and without hesitation. Kayo has known Mitsuko since they entered high school and initially felt envious of her strong resolve, which later turned into admiration. Their interactions reveal that Kayo is able to see Mitsuko’s more childish and genuine self beneath her mature persona. Kayo even gives her the nickname Micchan, and the narrative suggests that Kayo harbors quiet romantic feelings for her. This potential romance is implied to continue developing, as an epilogue image shows the two together, wishing the main characters well. Kayo also shares a friendly relationship with Harumi, who is the one to give her the nickname Glasses Senpai.

Throughout the story, Kayo demonstrates notable development in her confidence and understanding of others. Her quiet strength and perceptiveness allow her to offer thoughtful guidance at pivotal moments, making her an indispensable pillar of support. She displays a natural talent for reading between the lines, often noticing the unspoken feelings between Yuzu and Mei long before other characters do. While she lacks flamboyant physical abilities, her observational skills and empathetic nature are her most notable traits. Initially struggling with rule-breakers, her character evolves to show a more nuanced understanding of people, culminating in her supportive role for the main characters and her burgeoning romantic pursuit of the formidable Mitsuko.