A ruthless career criminal, Parker has almost no traditional redeeming qualities, aside from efficiency and professionalism. Parker is cold, methodical, and perfectly willing to commit murder if he deems it necessary to get what he wants. He does, however, live by one ethical principle: he will not double-cross another professional criminal with whom he is working, unless they try to double-cross him. Should that happen, Parker will unhesitatingly undertake to exact a thorough and brutal revenge.
Parker's first name is never mentioned in the novels, and there are many details about him which remain unknown. In fact, it is hinted throughout the series that the name "Parker" might itself be an alias.
In a 1981 introduction to a reprint of The Mourner (1963), Westlake's friend and fellow crime novelist Lawrence Block describes Parker as rare among anti-hero protagonists in that the character never develops a conscience. Block argues that novelists are generally "uncomfortable writing consistently from an antisocial perspective", and tend to soften such characters over time. However, " never turns honest, or finds God, or starts working as a secret agent for the government." According to Block a sign of Westlake's genius, and the key factor in the character's durability across the decades, was the realization that " mellow Parker is no Parker at all."
While in 1966's The Handle Parker's age is explicitly stated to be 38, Parker is, essentially, an ageless character — in the various Parker novels that were written and take place over a span of 45 years, Parker always appears to be somewhere around 40.
Physically, Parker is described in the opening paragraphs of The Hunter as "big and shaggy, with flat square shoulders... His hands, swinging curve-fingered at his sides, looked like they were molded of brown clay by a sculptor who thought big and liked veins. His hair was brown and dry and dead, blowing around his head like a poor toupee about to fly loose. His face was a chipped chunk of concrete, with eyes of flawed onyx. His mouth was a quick stroke, bloodless." When asked about who he would cast as Parker, Westlake stated: "Usually I don’t put an actor’s face to the character, though with Parker, in the early days, I did think he probably looked something like Jack Palance. That may be partly because you knew Palance wasn’t faking it, and Parker wasn’t faking it either. Never once have I caught him winking at the reader." In The Man With the Getaway Face, Parker has plastic surgery in an attempt to evade The Outfit's retribution, so he's no longer recognizable to most who knew him before, though his general appearance (and the impression it makes on others) seems to be largely unchanged.