Kokoro Yumemi, Teru Yumemi's energetic older sister and a third-grade student, frequently argues with her younger brother. Her first day at a new school saw her challenge and defeat the duelist Chishio in a "Creature Battle Duel," instantly claiming the title of the school's top duelist and holding the status of a Level 24 duelist.

Her involvement escalates when she and Teru encounter Rei Kukami, who is pursued by the organization Nest. Assisting Rei leads them to her grandfather, Kenshiro Kukami, who runs a dojo dedicated to teaching the Duel Masters card game. While Teru trains there to gain strength and protect friends, Kokoro's presence and formidable dueling skills remain integral to their dynamic. The siblings focus on protecting Rei and confronting Nest, with Kokoro participating in key conflicts.

Kokoro specializes in a Light Civilization deck. Her initial strongest card was Algros Crusos, Super Divine Dragon. After winning a training duel against Rei, Kenshiro rewarded her with the powerful Supernova Venus la Saint Mother, significantly enhancing her deck's capabilities.

An episode titled "Cocoro's Heart" specifically focuses on her character. Her role is consistently proactive, particularly in dueling challenges and supporting her brother and allies against antagonistic forces like Nest.