Kusanagi's Bio
(There are probably some spoilers here, so be careful.)

I shall give you the seven souls. Use the power of our family to become strong. Protect Princess Kushinada. Protect her from the human fools who would sacrifice her to banish us. Protect the Kushinada using the seven souls. Protect her!

—Yamata no Orochi, episode 2

Kusanagi [Kusanagi-no-tsurugi]: "grass parting two-edged blade;" the sword that was removed from the tail of Orochi by Susano-oh; originally the property of the Dragon-king of the Sea; later became one of the three emblems of Imperial dignity and could be wielded only by the Emperor of Japan

Mamoru: (Japanese) protector; guardian

When Kusanagi was still a little child, the Yamata no Orochi killed both of his parents, and gave Kusanagi the task of protecting the Kushinada from the humans who would sacrifice her in order to be rid of the Aragami. The Orochi then gave the child seven of his six mitamas (seeds), in the process creating a human-Aragami hybrid.

Because of these seven mitamas, Kusanagi's appearance as well as his abilities were very much changed. He can move faster than any human, and jump over incredibly long distances; he also has enhanced self-healing abilities. He has cat-like eyes, orange-tinted skin, green hair and curved, plant-like blades grow out of his forearms on command (and four eyebrows ^^). From episode ten on, he shows other sorts of abilities. For instance, using the mitamas on the back of either hand, he can shoot ki (energy) blasts, and—like the the blades in his arms—he can generate a pseudo-armor to protect his upper body (the growth of this armor also presents a great opportunity to see him without a shirt on).

Until the incident at the very beginning of the series, Kusanagi had spent his entire life protecting Kunikida Kaede, the Kushinada. However, after she disappears, the duty Orochi put on him forces him to seek out Kaede's twin, Fujimiya Momiji, who has lived separately from, and never met her sister. Kusanagi decides that he no longer wishes to have his life controlled by the Aragami, and decides to kill Momiji before the Aragami have a chance to sacrifice her themselves (using what is called a "ceramic field," the Aragami can harnass the power in the Kushinada's blood and kill her without her bearing any threat towards them). However, when he finds her, he finds two things standing in the way of his goal, 1) Momiji's resemblance to Kaede and 2) a government agency called the TAC (Terrestrial Administration Center). He tries again later but, instead of killing her, he saves her from Orochi. She returns the favor by stepping between him and the Orochi as the Aragami is about to kill him, and Orochi has no choice but to surrender his last mitama to her.

Later on in the series, Kusanagi finds himself falling in love with Momiji; after being faced with a several Aragami-induced visions of Kaede, he realizes that he can't live in the past. However, he still feels protective of Kaede, which is evident when she reappears after Susano-oh's birth. This doesn't stop him from attacking the god, or from fighting Murakumo, who is protecting the two of them.

After Momiji learns to use the Kushinada pulse (a melody that might bring about the end of the Aragami without her dying as well), the seed falls off of her, and Kusanagi takes it, thus becoming a "perfect soul" with the ability of flight and the strength to finally defeat Murakumo.