Juugo's classmate. Tensai is smart and into solving puzzles; she aspires to be a master detective and refers to herself as such. Though smart, she has trouble properly explaining her deductions as her thoughts go too fast for her to fully express them. She moves into the apartment next to Juugo's and becomes his neighbor. She knows that Jūgo is connected to Matsuri and declares him her rival, and thus frequently follows him and even occasionally wishes that events lead him to criminal activity. Her Nanana Collection is a charm which is rumored to have exorcising ability. Whenever she meets someone, after they give their name she asks for their "real name".

Nanana's Buried Treasure (龍ヶ嬢七々々の埋蔵金 Ryūgajō Nanana no Maizōkin, lit. "Nanana Ryūgajō's Buried Treasure") is a Japanese light novel series, written by Kazuma Ōtorino with art by Akaringo. Enterbrain has published seven volumes since January 2012 under their Famitsu Bunko imprint. A spin-off light novel series titled Ikkyū Tensai no Kiwamete Fuhon'i na Meisuiri (壱級天災の極めて不本意な名推理, lit. "Tensai Ikkyū's Reluctant Deduction") is also published under Famitsu Bunko, with the first volume released on August 30, 2013. A manga adaptation by Hitoshi Okuda began serialization in Enterbrain's Famitsu Comic Clear web magazine in January 2012. An anime television series adaptation by A-1 Pictures began airing from April 10, 2014 on Fuji TV's noitamina block.