Lucan is the Leader of the Order, a Gen One that lead the war against the otherworlders that started the race. He started the order with a bunch of Gen One warriors, including Tegan, so he could get rid of the rest of the ancients. He killed his father after he killed his mother.
Gabrielle is his breedmate. His son is Dorian. Marek is his brother.
He was first introduced in Kiss of Midnight. That is where he met Gabrielle because she took pictures of a Rogue attack at a night club. He is in every book of the series.