In manga Nanashi doesn't show to be a Key Box. People on Reddit theorized about because his/her name based in a numeral like Hachi(Eight)Maru and Go(Five)ku. But Ichigo has just one kanji in her name meaning " Strawberry" and Ryu maybe has another name. Nanashi means "Nameless". Maybe in the future he/she keep this name but altering the kanjis to fit something like Nana(Seven)Shi.
But know he/she doesn't fit the criteria (huge gravity or an uncorrupted White Samurai Soul).