Luke was a member of The Master's Order of Aurelius. He was very loyal to The Master, and was chosen to be the Master's vessel on the night of The Harvest. As his vessel, Luke could give power to The Master every time he fed on someone. If he fed enough, he would be able to free The Master from his dimensional prison. Although Luke fed on many people, The Slayer soon showed up, after fighting for a while, The Slayer distracted Luke and then staked him.
Luke is a fairly old vampire, probably about as old as Darla. This is evident in his strength, his loyalty to The Master, and the way he spoke. It's possible he was sired after Darla, and joined The Master's Order during the time she left if for Angel. He seemed to have higher authority than her, which is possible even if he was younger because he would have authority over someone who left The Order. If he was older then Darla, then he has never been seen in any flashbacks from the time Darla was in The Order, but this could be because he was off recruiting, just as Darla did for a while. His huge size and strength was probably why he was sired, and like Darla, The Master probably sired him personally.