The Order of the Green Hand was an ancient knightly order founded by House Gardener, whose sigil was a green hand. It is the only Knightly order found in the North.
It's current Lord Commander is Omer Manderly, who wields a considerable influence in the affairs of White Harbor.
Omer Manderly and his fellow Green Knights welcomed Lord Stark into White Harbor and gave him guest right. He then accompanied the Starks to the steps of New Castle and helped arrest his nephews Androw and Cerrick Manderly.
Omer Manderly, the Lord Commander