Kokoro Katsura, born November 28, 1993, is the 11-year-old younger daughter of Mr. and Manami Katsura and sister to Kotonoha Katsura. She stands 122 cm tall with black hair, gray eyes, and measurements of 53-47-57 cm. Her facial features resemble Kotonoha's, but her shoulder-length hair includes two strands tied with beads or ribbons. Kokoro's personality starkly contrasts with her sister's reserved nature; she remains peppy, talkative, and intensely curious. She actively frets over Kotonoha's limited social life and teases her about romantic interests, particularly Makoto Itou. Demonstrating advanced knowledge of intimacy beyond her sister's understanding, she expresses pity for Makoto upon learning his relationship with Kotonoha lacks physical involvement. Kokoro exhibits manipulative tendencies, like threatening to falsely accuse Kotonoha and Makoto of misconduct to force Makoto into playing with her or orchestrating their encounters. During the "Valentine Days" storyline, Kokoro joins a hot springs trip with Kotonoha and friends. She assists Kotonoha in creating homemade chocolate for Makoto using unconventional ingredients due to missing cocoa beans, playfully questioning Kotonoha about her Valentine’s plans. Her role expands in "Summer Days" and "Shiny Days," where she becomes a sub-heroine capable of a romantic relationship with Makoto, including depicted sexual encounters. These scenes were initially absent from "Shiny Days'" English release but restored via a patch. In the "Strip Battle Days" spin-off, an older Kokoro endures years of sexual abuse by Makoto, transforming her from cheerful to silent and reserved. She submits under coercion when threatened with the rape of Kotonoha's daughter. Across manga adaptations, Kokoro discovers Kotonoha bleeding from a stab wound and frantically calls Makoto. In any narrative where Kotonoha dies, including the "I Love You and Forever" ending, she leaves a drawing at Kotonoha's grave alongside her mother, expressing grief. Her name reflects her essence: "Kokoro" (心) meaning "heart, mind, and/or soul," and "Katsura" (桂) referring to a cinnamon or Japanese Judas tree.