Not much can be said with certainty about the god Susano-oh as he appears in Blue Seed. In Shinto myth, Susano-oh is a god of tempests and storms, one of the the three main gods created by Izanagi, the principal deity of Shinto, along with Amaterasu the goddess of the Sun, and Tsukiyomi the god of the moon. The most well-known mythological tale involving Susano-oh is the tale where he fought the great eight-headed serpent Orochi to save the daughter of an elderly couple. He accomplished this by getting the monster drunk on sake, and then beheading it while it slept, and from the serpent's body discovered the great sword Kusanagi (also known as the Ame-no-Murakumo). The girl he took as his reward and married, and she became known as the Princess Kushinada.
How much of this is true in the Blue Seed version of events is not quite known, though it is clear that something like it happened, for the Princess Kushinada's descendants, the mysteriously survived or ressurected Orochi, and Susano-oh himself all figure heavily into events. What is known is that Susano-oh himself longed to be reborn and take vengeance upon the people of Japan who had grown corrupt and lost their closeness with nature, and that the Orochi and its servants the aragami worked to bring this about.
Susano-oh, despite being a god, is a near helpless infant after his rebirth. His plan, as executed by his servant the Kushinada Kaede, was to travel to all the great spiritual sites of Japan, and thus mature on their power and connection with the soil. While the full extent of his true powers was unknown, it is unknown that Susano-oh has the potential to successfully carry out his plan of transforming every single human in Japan into plants, and his powers greatly surpass that of the Orochi Murakumo.