Originally, Kusanagi worked with the arigami. His job was to keep the Kushinada from being killed. According to legend, if the blood of the Kushinada was spilled, it would put the Arigami into an eternal sleep. The only way to nullify the effects of a Kushinada's blood is to kill her when she is in a special ceramic field.

When Kaede, the first Kushinada was destroyed, Kusanagi was supposed to protect her twin sister, Momiji. He attempted to kill Momiji in order to liberate himself from his Arigami 'masters'. He couldn't bring himself to do it. This might be because Momiji looked so simmilar to Kaede, whom Kusanagi had strong feelings for. Momiji ended up saving Kusanagi after a battle with Orochi, when she took a blow that was meant for him. (This was how Momiji recieved her mitama)

Later Kusanagi began to work against the Arigami by protecting Momiji from them and their ceramic feilds. Allthough he is not a part of the TAC, he ends up being very helpful, and often a necessary addition to the team. He has saved Momiji countless times.

Momiji has a major crush on Kusanagi, even though he makes fun of her. They make a cute (but strange) couple, don't you think?

Sort of off subject, but when Kusanagi comes in contact physically with Momiji, his seven mitama and her one react, giving them both the ability to fly (But only as long as they are touching). Later when Susano-oh's power grows and Momiji's Kushinada blood begins to get stronger, they can telepathically communicate as well, and Momiji learns how to 'sense' Kusanagi's energy.