Doyle is a seer, who receives prophetic visions from The Powers That Be, usually of people in peril. His half-Brachen demon physiology grants him the ability to shift from normal human to demonic appearance (red eyes and green skin adorned with blue spikes), in which he has heightened sense of smell and superior strength.
Originally, it was Whistler (ambiguous demon seen in Buffy's second season episode, "Becoming") who was to be a supporting character in the spin-off series starring Angel. Due to Max Perlich being unavailable, the character of Doyle was quickly created with similar character traits.
At the TCA Writer's Guild of America West party in 2000, supervising producer David Fury stated, "Joss has bandied about, 'I love the idea of putting a character in the main credits as one of the stars of the show and then kill him right off the bat.' But in the case of Doyle, he didn't want to kill off Doyle. It just became a situation. The work situation became difficult... It's hard enough to make a television show without the headaches."
In interviews before Quinn's death, creator Joss Whedon discussed plans for Doyle to return to the show as one of the season's big bad roles Template:Fact. Quinn died before anything could come of the talks.
Doyle was a series regular for part of the first season of Angel. He appeared in nine episodes overall. Allen Francis Doyle on Wikipedia