"Hawk's magical." You need psychotherapy, he can do that. You need somebody's leg broken, he can do that. You need to know where there's a good restaurant, he can tell you. He does whatever he needs to be able to do, and he does it without any particular concern about larger issues. He's the eminently practical man, and he is what Spenser might have been had Spenser grown up a minority figure in a majority culture. - Robert B. Parker explaining Hawk to actor Shiek Mahmud-Bey
Hawk appears in most of the Spenser novels to serve as Spenser's dark reflection. He started out to be Spenser's adversary in Promised Land, a man as tough and imposing the hero himself, but working as an enforcer for a Boston crime boss. In later novels, he worked for or with Spenser, at first for money, then later out of a form of friendship. Hawk is a larger than life figure, always dressed in the height of fashion, from his tailored leather trenchcoats to his hand-stitched boots. He drives high-profile Jaguars, and his guns, usually a .44 Magnum revolver and sometimes a shotgun, are often referred to as artillery.
Hawk and Spenser are mirror images of each other in most ways. Both were boxers, where they first met fighting in the same venues. Both spent some time in the military, Spenser in the Army, Hawk in the Foreign Legion. Both have chosen to be at the top of very violent professions. However, where Spenser, with his emotional attachments to the world around him, can indulge in higher level moral judgements, Hawk has been forced to be far more practical in order to pull himself alone from the ghetto of his birth. He has been forced by circumstances to shed his emotions to refine himself into a finely tuned killing machine. He is completely self-sufficient, requiring nothing from his environment and giving nothing back.
For Hawk, the world around him is either interesting or uninteresting, amusing or not amusing. He doesn't enjoy killing, but neither does he dislike it. Pragmatic to the core, he will kill when it will solve a problem. Mercy won't stay his hand, only cold logic. This has put him at odds with Spenser on more than one occasion; Spenser's moral code only allows for killing as an extreme last resort, and for him looking the other way while Hawk takes care of it is tantamount to doing it himself. Hawk, on the other hand, feels that Spenser has far too many rules complicating his life, rules that may one day get the private eye killed.
In the Spenser: For Hire television series, Hawk was played (perfectly, in my mind) by Avery Brooks. His portrayal is what first got me interested in the Spenser novels, and upon reading them, I found that Parker's descriptions of Hawk fit the actor perfectly. And Brooks's deep, almost James Earl Jones deep, voice was ideal for the dialog. ABC tried to cash in on Hawk (and Brooks's) popularity with the spin-off series A Man Called Hawk, but the show was a dismal failure. Because of his "magical" nature, as Parker puts it, Hawk cannot be a stand-alone character. He does not, for example, have a home, and his emotionless persona does not lend itself to very many personal attachments. He basically exists only as we see him through Spenser's eyes, and all but discorporates when Spenser's not around. In A Catskill Eagle, Parker calls him a phantom, the "ghost who walks."
Brooks went on to play Hawk in the Spenser made for tv movies from Lifetime, and was succeeded by Shiek Mahmud-Bey in Small Vices and, surprisingly enough, Ernie Hudson in Walking Shadow. While they were not as close to the Hawk archetype as Avery Brooks was, both men did a good job with the role, primarily as a result of showing a real interest in how Robert Parker viewed the characters. The only real problem I saw with Ernie Hudson was that to this day I cannot look at him without seeing him in his Ghostbusters role. He's hopelessly typecast in my mind.
A complete list of Parker's Spenser novels can be found here.
Sources:
Robert B. Parker's Spenser novels
Bullets and Beer - http://www.mindspring.com/~boba4/
http://www.thrillingdetective.com/non_fiction/e001.html