The origins of the Green Knight are unknown. He/it appears to be a mystical entity somehow embodying the spirit of the British Isles, which is linked to or based at the Green Chapel of Avalon on Otherworld. The Green Knight is engaged in an endless war with his/its opposite number, the Red Lord, and his/its servants the Bane, and periodically empowers human champions to fight the Bane on his/its behalf. In the past, these champions included King Arthur Pendragon and the knights of the Round Table, and in recent years the Knight has bonded the same 'Pendragon spirits' which once belonged to them, to a number of others including the superhero Albion (who became a Pendragon during the first World War) and, more recently, the Knights of Pendragon. Whether the Green Knight has any connection to the Pendragon spirit which was given to the Black Knight (Dane Whitman) is unclear, though it seems likely.
The Green Knight was seemingly destroyed by the Skrulls when they invaded Avalon, but was later restored by Pete Wisdom and MI13.