Kazuki Kosuda, Takashi Kosuda’s older sister, shares their family home while pursuing college studies. She boasts short dark red hair, brown eyes, a slender, curvy frame, and prominently large breasts. Playfully teasing yet deeply caring toward her brother, she flaunts her unselfconscious nature by lounging in revealing attire at home, indifferent to his flustered reactions. Her athleticism shines through weightlifting and motorcycle hobbies.

Quick to detect Yamada’s crush on Takashi during early visits, Kazuki mentors both to nurture their budding romance. Childhood friend Mayu Miyano once dubbed her “the Prince” for her bold, boyish charm, which initially confused Mayu into harboring a crush. Kazuki’s dynamic role weaves affectionate support with mischievous ribbing, steering the central relationship forward. When probed about her possible Eros Deity—a narrative symbol tied to libido—she deflects with a cryptic “secret.” Her presence consistently catalyzes the romantic plot through guidance and playful meddling, maintaining a steady persona across official narratives.