Once the Aragami had reawakened, they began to take steps to ensure their continuted survival. With that in mind, Orochi sought out a family that had been attempting to sacrifice the Kushinada and put the Aragami to sleep again. Orochi killed the parents...but for their baby son, Orochi had a special plan. Implanting within him seven mitama, seven Aragami souls, Orochi charged little Mamoru Kusanagi with the task of watching over the Kushinada and ensuring that no harm would come to her...until they were ready.

So even as he grew, Mamoru kept a watchful eye over little Kaede Kunikida, watching her grow and develop. He didn't know it then, but watching Kaede grow like this would stir up emotions later on he would have trouble controlling. For Kusanagi was not happy with his destiny. He sought freedom, and to do that, he thought, he would have to kill the Kushinada and let the pulse lock his masters in sleep.

Mutated as he is by the seven mitama on his body, green blood flows in Kusanagi's veins, and he has the ability to transform his body somewhat. He can make blades sprout from his arms, and he has incredible physical ability, including his penchant for long and high jumps. However, without an eighth mitama, he lacks the strength or the abilities to take on Murakumo. But Kusanagi is not one to give up easily, and his unusual connection to Momiji (she and he both received their mitama from Orochi) will play a crucial role in the late stages of the conflict.

As his name implies, "Mamoru" means protector, while "Kusanagi" comes from "Kusanagi-no-tsurugi", the Grass-Cutting Sword, a blade supposedly given by the Japanese gods to the emperor.

-taken from Absolute Anime