Yoshiko Koizumi, an honor student in Shōta Ōhara’s elementary school class, stands at 140 cm with striking black eyes and a signature large hair ribbon. Residing with her parents, Mr. and Mrs. Koizumi, an unnamed uncle, and her ghostly judoka companion U-ko, she navigates a life blending ordinary routines and supernatural ties. Her days involve shared meals with family and U-ko, lengthy phone conversations, and occasional participation in school events, such as cheering for Shōta during a baseball game where he teams up with the mischievous ghost Q-Taro.

Yoshiko fosters close friendships with classmates like the affluent Kizao Kiza and academically gifted Hakase, while remaining oblivious to the unrequited affections of Shōta and neighborhood bully Tsuyoshi "Godzilla" Saigō. Her interactions with Q-Taro and other spirits are marked by casual familiarity, though she stays peripheral to their chaotic antics. While consistently supportive of peers, she prioritizes maintaining friendships and school life over deeper entanglement in supernatural disruptions.

Across official media, her portrayal remains steady: a socially active, grounded student anchored in everyday challenges and connections, with no expanded backstory or significant evolution beyond her established role.