Karas (鴉 karasu lit. Crow) is the guardian of cities worldwide. Each Karas has an actual body and a Yurine.
A Yurine activates her Karas' Spirit powers. Until Eko was killed by Otoha, Eko was seen as the strongest Karas in Japan and had killed several Karas in his life-time.
Although every city is only supposed to have one Karas, Shinjuku had two. The reason for this being that a Karas is supposed to protect the city, and if they stop doing so, a new Karas is born. Eko stopped protecting the city and eventually tried to take it over. Otoha was selected as the cities' last hope and the Karas that would destroy Eko.