In professional wrestling, a heel (also known as a rudo in lucha libre) is a wrestler who portrays a villain or a "bad guy" and acts as an antagonist to the faces, who are the heroic protagonist or "good guy" characters. Not everything a heel wrestler does must be villainous: heels need only to be booed or jeered by the audience to be effective characters.
To gain heat (with boos and jeers from the audience), heels are often portrayed as behaving in an immoral manner by breaking rules or otherwise taking advantage of their opponents outside the bounds of the standards of the match. Others do not (or rarely) break rules, but instead exhibit unlikeable, appalling and deliberately offensive and demoralizing personality traits such as arrogance, cowardice or contempt for the audience. Many heels do both, cheating as well as behaving nastily. No matter the type of heel, the most important job is that of the antagonist role, as heels exist to provide a foil to the face wrestlers. If a given heel is cheered over the face, a promoter may opt to turn that heel to face or the other way around, or to make the wrestler do something even more despicable to encourage heel heat.
In "local" wrestling e.g. American wrestling, it was common for the faces to be "local" (e.g. Hulk Hogan) and the heels to be portrayed as "foreign" (e.g. Alberto Del Rio, Ivan Koloff, The Iron Sheik, Rusev, Jinder Mahal and Muhammad Hassan).
In the world of lucha libre wrestling, most rudos are generally known for being brawlers and for using physical moves that emphasize brute strength or size, often having outfits akin to demons, devils, or other tricksters. This is contrasted with most heroic técnicos that are generally known for using moves requiring technical skill, particularly aerial maneuvers.
Common heel behavior includes cheating to win (e.g. using the ropes for leverage while pinning or attacking with foreign objects while the referee is looking away), employing dirty tactics such as blatant chokes or raking the eyes, attacking other wrestlers backstage, interfering with other wrestlers' matches, insulting the fans or city they are in (referred to as "cheap heat") and acting in a haughty or superior manner.
More theatrical heels would feature dramatic outfits giving off a nasty or otherwise dangerous look, such as wearing corpse paint over their faces, putting on demonic masks, covering themselves in dark leather and the like. Gorgeous George is regarded as the father of the wrestling gimmick and by extension the heel gimmick. Starting in the 1940s, he invented an extravagant, flamboyant "pretty boy" gimmick who wore wavy blonde hair, colorful robes and ritzy outfits and was accompanied by beautiful valets to the ring for his matches. The crowd widely jeered his persona and came out to his matches in hopes of seeing him defeated. He in turn relished in it and exploded in to the one of the most famous (and hated) heels not only of his era, but of all time. Another example of a dramatic looking heel is the wrestler The Undertaker (in his heel roles). During his period in The Ministry of Darkness, he undertook performances where he would appear as a priest of the occult in a hooded black robe and sit on a devilish throne.
Occasionally, faces who have recently turned from being heels still exhibit characteristics from their heel persona. This occurs due to fans being entertained by a wrestler despite (or because of) their heel persona, often due to the performer's charisma or charm in playing the role. Certain wrestlers such as Eddie Guerrero and Ric Flair gained popularity as faces by using tactics that would typically be associated with heels, while others like Stone Cold Steve Austin, Scott Hall and more recently Becky Lynch displayed heelish behavior during their careers yet got big face reactions, leading them to be marketed as antiheroes.
On other occasions, wrestlers who are positioned as faces receive a negative audience reaction despite their portrayal as heroes. An example is Roman Reigns, who in 2018 was a top face in WWE, but got booed in his matches while his opponents got cheered regardless of them being face or heel, due to perceived favoritism from WWE bookers and executive and a lack of character development. Such characters often (but not always) become nudged into becoming villains over time or retooled to present a different public image, such as The Rock's heel turn from a clean-cut face to self-absorbed narcissist in the heel Nation of Domination stable. The term heel does not describe a typical set of attributes or audience reaction by itself, but simply a wrestler's presentation and booking as an antagonist.
Depending on the angle, a heel can act cowardly or overpowering to their opponents. For instance, a "closet champion" in particular is a term for a heel in possession of a title belt who consistently dodges top flight competition and attempts to back down from challenges. Examples include Seth Rollins during his first WWE World Heavyweight Championship run, Charlotte during her Divas/Raw Women's Championship reign, the Honky Tonk Man during his long Intercontinental Championship reign, Tommaso Ciampa during his NXT Championship reign and The IIconics during their WWE Women's Tag Team Championship reign. This helps to affirm the intended kayfabe reactions that the face(s) that said heel is feuding with is/are more deserving of the title. Heels may in fact beg for mercy during a beat down at the hands of faces, even if they have delivered similar beat downs with no mercy, with Ric Flair in particular being well known for this. Other heels may act overpowering to their opponents as to play up the scrappy underdog success story for the face instead. Brock Lesnar has played heel in both capacities, but has become quite famous (either as a heel or a face) as an almost-unstoppable machine who can take down anything in his path.