Who is Luke from City of Bones and what is their importance?

Luke is a werewolf leader, and has been Clary's 'Uncle Luke' since she was a little girl. Clary's mother and Luke were both a part of Valentine's Circle, but when Jocelyn left the Circle to take Clary and run to New York, Luke followed to keep them safe. Luke was once Valentine's best friend. He has been in love with Jocelyn for years but never acts on that love.