First Appearance: Marvel Comics #1 (November 1939).
Golden Age Appearances: Marvel Comics #1, Marvel Mystery Comics #2-79, Sub-Mariner #1-21, All-Winners #1, Mystic Comics v2 #1-3, Daring Comics #10.
Modern Appearances: Avengers #97, Marvels #1, Marvel Super-Heroes v3 #7, USAgent #1-4, Incredible Hulk #432, Marvel Super-Heroes Fall 1991 Special, Captain America #442, 4 #1, Citizen V & the V-Battalion v1 #2, v2 #1.
Dates Active: 1939-present (sorta--see below).
The Angel was Angelus, a vampire who--oops, wrong Angel. The Angel was Thomas Halloway, a suave private eye with a heavy resemblance to the popular pulp character The Saint; according to the Angel's creator, Paul Gustavson, the resemblance was not coincidental. The Angel's origin is interesting and not a little pulp-influenced. According to a text piece in Marvel Mystery Comics #20, Halloway was the son of a prison warden; his mother died in childbirth. The warden raised Thomas in isolation, in the prison itself, having him be instructed by various experts who taught him "everything." Thomas was also taught by the prisoners, who passed on the secrets of the underworld. Thomas' one friend in his youth is Bob Soler, and when Bob and Thomas discover that one of the prisoners is about to be electrocuted they save him. For this deed Thomas is dubbed an "angel" by the inmate. Marvel Mystery Comics #17 & 18 feature one Dr. Lin, a scientist who studied esoteric phenomena such as Tibetan lycanthropy and who was apparently the Angel's mentor. And Marvel Mystery Comics #75 claims that Holloway is a distant cousin to the British Harmon family.
The Angel had no super powers per se; generally he was simply a suave private detective who dealt with a variety of menaces, from gangsters to crazed pygmies. He did have certain marked advantages over ordinary men, however. His upbringing taught him...well..."everything." He's been shown to be fluent in a Greek dialect that died out four millennia ago, he is familiar with a wide range of cults and cultures, and he is a "brilliant doctor." As well, after helping helping a 4000-year-old woman escape from an underground city he is given "the cape of Mercury," which enables him to fly. (The Angel's ability to fly was shown only intermittently over the issues.)
In the modern era it was revealed that there were two brothers who, in the Golden Age, were the Angel. (This is an clear retcon, as the text piece makes no mention of Thomas having any brother.) One--and since I don't have the Hulk issues I don't know which one--was shown in Incredible Hulk #432 and Marvel Super-Heroes v3 #7 as a transient hanging out with the Abomination in the sewers of New York City; this Angel was killed by the villain Zeitgeist in Captain America #442. The other brother was shown in USAgent #4 to have become the backer and funder of the mask-killer Scourge, as well as a possible pawn of the Red Skull. After a mission in which an innocent bystander was killed, the Angel decided that he couldn't risk the death of innocents, and so he began looking for a way to atone for his guilt; he decided the best way he could do this was to kill supervillains. Marvel Universe Masters, however, implies that the Red Skull subverted only one Scourge, rather than all of them, so perhaps the Angel wasn't a pawn of the Skull after all. The Marvel Super-Heroes Fall Special noted that his arch-enemy was "The Stinger." Finally, as Ronald Byrd points out, Marvels #1 states that the Angel was active even before the debut of the Human Torch (I), making the Angel the first Golden Age hero.
Most recently Citizen V & the V-Battalion v1 #2 has shown that the Angel was a founding member of the Penance Council, the leaders of the V-Battalion. Citizen V v1 #1 established that the Angel was a member of the All-Winners Squad. Citizen V & the V-Battalion v2 #1 showed the Angel attending the funeral of the Union Jack (II). How this gibes with the pre-established post-war history of the Angel is something we'll have to hope for a future story to reveal.