Doyle was "the first soldier down" of A.I., having sacrificed himself to save a clan of demons as atonement for the sins of his past.
Doyle was born in Ireland to a human mother and a Brachen demon father. Doyle never knew his father or anyone on that side of his family, and his own demonic genes didn't physically manifest themselves until he was 21 years old. At the time, Doyle was a third grade teacher and soup kitchen volunteer married to a woman named Harriet, the love of his life. He didn't take the news of his demonic heritage well, in spite of Harriet's acceptance of his other side, so his marriage ultimately disintegrated. Doyle hid behind the flimsy veneer of a ne'er-do-well hustler and con artist, seemingly more interested in his next drink than with helping others. Later, he was approached by a fellow Brachen demon, Lucas, who told Doyle that The Scourge, a militant group of pure blood demons, was after all half-breeds and begged for Doyle's help.Doyle turned Lucas away, believing that this wasn't his problem. Soon after he received a vision, which he described as "splittin' migraines that come with pictures," in the throes of which he thought he was having a stroke. The vision showed him a group of massacred Brachen demons. Doyle searched the city to find out if what he had seen was real—it was. These visions, which come from The Powers That Be, are what led Doyle to Angel. As Doyle says, "We all got something to atone for," and therefore the two join forces to fight evil in Los Angeles. Some point around that time, he came to own an apartment. Once Cordelia Chase joins the team, Angel Investigations is officially formed.
Doyle soon fell in love with Cordelia, but was afraid she would reject him upon finding out about his demonic heritage, a belief reinforced by Cordelia's open black-and-white view of demons being evil. He also forms a close, brotherly bond with Angel. Despite Doyle's reluctance to discuss his past, Angel and Cordelia learned more about him when Harriet returned to his life, seeking a divorce so she could marry an Ano-Movic demon named Richard Straley. Harriet later called the marriage off after learning that obtaining the blessing of the Straley clan would require Richard to kill the ex-husband of his intended bride by eating his brain.
Doyle's past again came back to haunt him when the Scourge returned, threatening the Listers, another tribe of demons regarded as half-breeds. To save their lives as well as those of Angel and Cordelia, Doyle sacrificed his own life to disable the Beacon, a device which could destroy any being "tainted" with human DNA, leaping on to the Beacon and pulling out a vital power cable, being disintegrated by its energy output just as he deactivated it. In doing so, Doyle fulfilled the Listers' prophecy of the "Promised One", a being purported to save them from the Scourge in the last days of the 20th century. Before Doyle died, he shared a passionate kiss with Cordelia, who had only recently learned of - and accepted - Doyle's demonic heritage, and lamented that they would never know if she could come to fully love him as a half-demon. As the two kissed, his visions passed on to her.
After Doyle's death, Angel confronted the Oracles, asking them to rewind time to give him a chance to resurrect Doyle, but they refused, as doing so would render Doyle's very act of sacrifice and redemption meaningless.
In an alternate reality, Cordelia never joined Angel Investigations and Doyle passed his visions to Angel before dying. This eventually resulted in Angel having a mental breakdown, unable to handle the strain of the visions, especially in absence of Cordelia's humanizing influence, with only the alternate Wesley - here lacking one arm - and Gunn for company.
When Lindsey McDonald returned to Los Angeles in 2003, he assumed Doyle's identity in an ultimately failed attempt to convince Angel and Spike that the latter was the subject of the Shanshu Prophecy; both Angel and Cordelia were outraged and disgusted that Lindsey stooped so low as to abuse Doyle's name and legacy.
Personality and traits
Doyle's most defining trait was his discomfort with his Brachen demon heritage, preferring to remain human at most times and only using his demonic powers when absolutely necessary.
Prior to meeting Angel, Doyle largely lived as a hustler and con artist as a means to cover up his guilt over failing to save Lucas and his clan from the Scourge. Despite this and his mission, he remained a down-to-earth individual, even encouraging Angel to reconnect with the people he was protecting.
Powers and abilities
Doyle received prophetic visions from The Powers That Be, which includes images of people in peril, names, places in which evil was present or threats the Powers wanted to be dealt with. These visions also caused Doyle great pain, though his half-demon physiology prevented his brain from suffering any long-term physical damage.
His half-Brachen demon physiology grants Doyle the ability to shift from human to demonic appearance - red eyes and green skin adorned with blue spikes - in which he had a heightened sense of smell as well as superior strength, speed, stamina, and dexterity, the last of which allowed him to twist his head around in such a manner as to fake a broken neck, something that allowed himself and Angel to enact a plan against the Scourge. Regardless, Doyle rarely assumed his Brachen visage, preferring to remain human due to his discomfort with his demon heritage.