Not much was really known about Doyle, what we did find out was that he was half human and half Demon who is sent to assist Angel and also guide him from the very beginning. He would however rather spend his evenings in a bar than risking his own life. But having his 'Gift' also gives him the sense of duty to help save people he sees in his visions. Given the gift of visions also leaves him in pain afterwards, he has vicious headaches, but they also give him a head start into what evil lies ahead. He helps Angel to try and take his mind of Buffy by being his crime fighting side-kick and most of all his friend.
We do know that he was previously married `the bachelor party` and escaped his brains being eaten in this episode. During the short time that Doyle worked with both Angel and Cordelia they became very close, Doyle especially to Cordelia, although his advances where always avoided or put to the back of her mind. Never really admitting to herself that maybe they could of had a future together.
Angel, Cordelia and Doyle encounter a tribe of pure Demons called the Scurge, they track and kill half breeds of all species. The Demons that Doyle, Cordelia and Angel are trying to save are found passage on a boat, while Doyle and Angel are caught by the Scurge, Angel acts as if he is evil and breaks Doyles neck in front of them asking to join them and help them kill the half breeds. They agree and he follows them to their lair where they are preparing the ultimate device to kill all things that have a single drop of human blood in them. They demonstrate this with a human who has also told them where to find the Demons that are ready to leave on the boat.
After all are trapped including Angel, Doyle and Cordelia on a boat with the device activated. Angel begins to say his goodbye's to his friends as he knows that he must sacrifice his live to save the ones he cares about, Doyle however hits him and he plunges to the floor a few feet below. Doyle turns to Cordelia and kisses her goodbye, and then just before Angel can get there to stop him he jumps over to the device.. Angel and Cordelia can only watch as their much love friend gives his life to save them all - disintegrating before their eyes but succeeding in turning the device off........
Doyle shall be sadly missed.
Doyle: "It's not all about fighting and gadgets and such. It's about reaching out to people. Showing them that there's love and hope still left in this world.
Homeless Woman: Got any spare change? "
Doyle: "Get a job, you lazy sow."