A tweener is a character who is portrayed as being morally neutral or ambiguous (that is, they are between a face and a heel). Sometimes referred to as greys. Another term to describe this type of character is an anti-hero.
Many followers of professional wrestling, especially within the smark community, consider Diesel, the character portrayed by Kevin Nash in the WWF during the 1990s, to be the first truly prominent tweener in American wrestling, in the sense of being portrayed as such. However, this distinction could arguably be given to Jake "The Snake" Roberts, often being cheered despite his dark character. Allen Coage, best remembered by wrestling fans as "Bad News Brown", is another early example of a tweener. It should be noted that Ric Flair, in all of his Face Runs, still performs dirty tactics.
During the mid to late 1990s, tweener gimmicks became increasingly popular in ECW, the nWo era in WCW, and Attitude Era in the WWF. These saw a number of crowd favorite faces, rather than become traditional faces, instead adopt heel tactics. Some examples of such are the nWo Wolfpac, D-Generation X, the A.P.A., Lita, 3Live Kru, The Undertaker, Kane, Big Show, Steve Austin, The Rock, Sting, Kurt Angle, Cryme Tyme, Samoa Joe, Carlito, Eddie Guerrero's "Lie, Cheat and Steal" gimmick, Mickie James' simultaneous obsession with face Trish Stratus and hatred for Trish's friend Ashley Massaro (although James has since turned full-blown heel and then face again), and John Cena in the months immediately after his late-2003 face turn. Usually, tweeners will attack both heels and faces simultaneously. There are occasions where they work together better with faces rather than heels in tag team situations and give meaning to the saying, "CLEAN HOUSE." For example, Stone Cold Steve Austin and The Rock, despite being rivals, have been able to work together effectively in tag-team situations. Also, during Kurt Angle and Chris Benoit's run as a tag team in 2002, they played tweener characters, feuding with faces Edge and Rey Mysterio, and heels Los Guerreros. Carlito can also be considered a tweener as he is popular with fans but will attack anyone that gets in his way or that he feels is a threat to him (not caring whether they are face or heel) as well as displaying immoral acts such as spitting on defeated opponents. Hardcore Holly is often considered an excellent example of a tweener as well—his character has rarely portrayed a full-blown face or heel since 1999.
Lex Luger was portrayed as a tweener in later 1995 and early 1996 when he immediately jumped from the WWF to WCW in the early days of the Monday Night Wars, not making his allegiances known (neither siding with Hulk Hogan and his allies or Kevin Sullivan and his Dungeon of Doom, who were Hogan's main rivals at the time) except for his long-time friendship with Sting. He merely claimed that he wanted to stake his claim at Hulk Hogan's WCW World Heavyweight Title, though he would later turn heel and join the Dungeon of Doom.
In the days when the NWA World Heavyweight Champion traveled the country in non-televised events, he was a "tweener" in that he was a "heel" when up against the local "face" and a "face" when he faced a "heel". Finlay also is an example of a tweener since he has challenged King Booker who is a heel.
In 2016, Sheamus was chosen to pair with Cesaro as tag team by Mick Foley. Since that time, Sheamus gradually got support by audience dispite his slightly-remaining heel personna, which makes him a tweener.