Kokoro Akechi leads Genius 4, a police unit investigating phantom thief incidents. She possesses a bossy personality and frequently disparages the Milky Holmes group following their loss of supernatural powers. Kokoro vehemently objects to being called "Kokoro-chan," especially by Sherlock Shellingford, often striking her with a golden mask in retaliation. This mask connects to her supernatural ability to manifest protective golden barriers. She repeatedly claims to have graduated from Harvard University at age 13 and asserts an IQ of 1300. Her name references the fictional detective Kogoro Akechi from Edogawa Rampo's works. Physically, she has blonde hair, stands 139 cm tall, weighs 30 kg, and has a birthday on May 1st.

During the Summer Special, Kokoro faces a forced career crisis when kidnapped by Irene's father, who aims to make her an idol to fund his failing business. Initially resistant, Kokoro eventually yields, attempting the idol lifestyle under the stage name "Kokoron." She finds idol work grueling and unsatisfying. Her absence leaves Sherlock Shellingford lonely. Recognizing Kokoro's dissatisfaction and Sherlock's loneliness, Henriette stages the theft of a statue. This fabricated crime reignites Kokoro's passion for detective work, prompting her to abandon the idol career and return to leading Genius 4, reaffirming her dedication to law enforcement.

Kokoro reappears in the spin-off "Tantei Kageki Milky Holmes TD," continuing within the franchise's continuity. She persists as the leader of Genius 4, continuing her comical clashes with Sherlock over the nickname "Kokoro-chan" and her habitual citations of her claimed academic achievements. Her role consistently involves confronting phantom thieves, showcasing her characteristic intellectual arrogance and leadership of the police unit.