Sumireko Sanshokuin, nicknamed Pansy, is a second-year high school student and library helper. Her nickname originates from rearranging the kanji in her name, removing "院" and "子" to form "三色菫," meaning Pansy, reflecting the flower.

At school, Pansy deliberately maintains a plain, unremarkable appearance with chest-length black hair in low twin braids, thick square-rimmed glasses, and a gloomy demeanor. This disguise hides her true identity as a strikingly beautiful individual with hip-length hair and a notably larger bust size, revealed only selectively outside school or to specific people. Her attire shifts dramatically for events: a red, low-cut floral-patterned dress with white gloves symbolizing pansy colors (yellow for memories, white for love, purple for devotion) during the Hundred Flower Festival's Kabuten dance, and a distinctive white skirted swimsuit at the pool.

Pansy possesses keen observation, sharp intellect, and a reserved nature. She is generally polite and kind to peers but directs a critical, manipulative attitude specifically toward the protagonist, Amatsuyu "Joro" Kisaragi. She demonstrates genuine care for close friends like Himawari and Cosmos. Highly perceptive, she discerns others' true intentions and subtly guides situations to her advantage. She openly admits to uncontrollable stalking of Joro, using it to gain insights into his behavior.

Her background involves significant middle school trauma. Her undisguised beauty attracted unwanted attention until friends, including Yasuo "Hose" Hazuki, protected her. Pressured by friends Momo Sakurabara and Runa Kusami, she reluctantly entered a relationship with Hose, causing deep discomfort. To escape this, she transferred to Nishikizuta High School and adopted her plain disguise. A pivotal moment occurred a year prior at a regional baseball playoff. Observing Joro (then using his real name Amatsuyu Kisaragi) selflessly waiting with fried skewers for his friend Taiyo "Sun-chan" Ōga after a loss moved her, sparking immediate romantic attachment. At the stadium exit, she encountered Sun-chan, offering either encouragement or criticism (accounts vary), inadvertently causing him to develop feelings for her. This initiated her covert observation of Joro.

Her development centers on preventing Joro from adopting Hose's negative traits like manipulation and an inferiority complex. She forces Joro to confront his flaws by maintaining her dual identity and guiding his interactions. Key arcs illustrate this: during the Kabuten Arc, she saves the festival dance when Sun-chan becomes unavailable, showing increased openness. In the Tsubaki Arc, her conflict with Joro over his motivations for replacing a ruined book emphasizes her value of sincerity over self-serving actions. Her past resurfaces in the Toshobu Arc with middle school acquaintances like Cherry and Tsukimi, compelling her to reconcile her history with current relationships.

Beyond manipulation and observation, Pansy excels at disguise, seamlessly switching personas. She is a proficient baker, often preparing library snacks for Joro or cookies for study sessions. Her preferences include classical literature (favoring Natsume Soseki's *Kokoro*), dislike of math, enjoyment of ginger pork, aversion to konnyaku, self-identification as a "mountain girl," prioritizing the present, and aspiring to marriage.