Yakumo Koizumi, a foreign-born English literature lecturer at Tokyo Imperial University, originates from Greece and was raised in Ireland. He adopted his Japanese name out of deep appreciation for the culture. His appearance features heterochromia, and he typically wears glasses.

Koizumi possesses extensive knowledge of Japanese spirits, mononoke, the focus of his personal research. This expertise drives him to investigate supernatural occurrences, identifying spirits like the onmoraki and warning others of possessions. Outwardly friendly and cheerful, he harbors a calculating, manipulative nature. He interacts dramatically and familiarly, often using flamboyant compliments, potentially to gather information on spirits.

His professional role contrasts with his personal obsession, dedicating significant time to collecting mononoke-related information. Multilingual, he enjoys sea bathing. Socially, such as at the Rokumeikan ball, he displays gentlemanly behavior yet intervenes abruptly in conversations. He strongly disapproves of Mei forming relationships with those he deems unsuitable.

Regarding mirror-related events, he contributes his spirit world knowledge alongside others. His dialogue frequently references destiny and supernatural connections, emphasizing his belief in the intertwined spirit and human worlds. Quotes include dramatic declarations like, "What a fortunate occurrence to see you again in such a place. I cannot help feeling it is destiny!" and protective warnings: "Unless you listen to me, I will need to... you will put me in a position where I will have to restrain you by force." He openly states, "Unfortunately, I am not a mononoke. I am just a foreigner who loves Japan very much. I am no prey for you."