Itsuki died in a traffic accident when she was still in high school. She came back as a Shikabane Hime because of her regret of not doing anything in her life. Itsuki's weapon of choice is a pair of pistols.
Itsuki's contractor monk is Takamasa Sougi. She picked Takamasa because she thought he was the strongest monk and would help her get to Heaven the fastest. The two have very strong feelings for each other but can't express those feelings or act on them because it is against the rules for a monk to love a Shikabane Hime. Himes are technically dead, therefore defiled, and monks are supposed to be pure. Monks are told to not even think of their Himes as human. Towards the end of the series they openly admit their feelings towards each other.
Itsuki at school with Ouri
Itsuki goes undercover as a high school student where she meets Ouri. Unbeknownst to him, she is there to keep an eye on Ouri, who has been seen interacting with Shikabane, Shikabane Himes and their monks. Itsuki's true identity is revealed when she saves Ouri from the Traitor Monk, Akasha. Itsuki and Takamasa continue to help Ouri and Makina through out the series.