From Perpetual Dreams:

7th grader Oshiro Shinobu, the daughter of one of Japan's old family lines, was separated from her parents due to certain circumstances when she was only six. Because of this she now lives with one of her distant relatives, Kouda Shougo, who is a teacher the at the school she now goes to. Apparently, a childhood friend by the name of Oku Saizou is transferring into Shinobu's class, but wait... This timid and hesitant boy is actually a ninja who serves Shinobu and he has transferred for the sole purpose to protect her...?!