Tachibana runs the SM store that Kaoru frequently shops at. She has vast knowledge of SM and teaches Nana about various things involved in the life style. Often seen being sort of a mentor to both Nana and Kaoru.
Little is said about Tachibana in the main series of Nana to Kaoru; however, in the spin off "Arashi" more is revealed. She majored in Japanese history at Todai, studying culture and customs since the Meiji period. She even went to grad school.
It is also revealed that her Master is Sarashina-sensei, Kaoru's idol. She often helps him with writing his books.