Hawkeye (Clint Barton; also known as Goliath and Ronin) is a comic book superhero that appears in Marvel Comics. Created by writer Stan Lee and artist Don Heck, the character first appeared as a villain in Tales of Suspense #57 (Sept. 1964) and later joined the Avengers in Avengers #16 (May 1965), and has been a prominent member of the team ever since. In addition to the mainstream Marvel Universe (designated Earth-616) version of the character, Hawkeye has numerous alternative versions throughout the Marvel Comics multiverse.
Ultimate Hawkeye
The Ultimate Marvel imprint title the Ultimates features a version of Hawkeye who uses a cover story of being a former Olympic archer to hide his conviction of an unexplained murder charge when he was recruited by Nick Fury into the Ultimates program, a government sponsored program made up of humans with extraordinary abilities and super-human operatives. During his time with the Ultimates, most, if not all, of his missions were of the Black Ops variety in which he partnered with the Black Widow.
Seeking revenge, Hawkeye tracked down the Black Widow, passing herself off as a wounded civilian after the invasion, and executed her. As addressed in "Ultimates 3", it was shown that the events during the invasion left him brooding and emotionally devastated, seeming almost to have a "death wish," and his brash, morally ambiguous and borderline sadomasochistic behavior became even more of an issue.
Whether or not his teammates know of his augmentations is unknown, with the exception of Nick Fury, who has confirmed his knowledge, but it would appear that, in Hawkeye's favor, they remain unaware of his full abilities. It has also been remarked that before the "Ultimate Hawkeye" miniseries, he claimed to need to wear corrective goggles to see properly, but it appears that that too may have been a ruse.
What if? Dark Reign
In What if? Dark Reign #1, Clint Barton succeeds in killing Norman Osborn. The superhero community then hunts him down for his crimes, while the public and the government turn completely against superheroes. While Mockingbird gives him what he needs to escape, he ends up being shot and killed by a mentally unstable man who wants to prove a point against superheroes, leaving Victoria Hand completely in charge of HAMMER.