With the long-awaited fifteenth season of It's Always Sunny in Philadelphia currently airing on FX, the show is now the longest-running live-action sitcom in the history of U.S. television. During its decade and a half on the air, the series has introduced some fantastic characters, many of whom viewers will be hoping to see return at some point during season 15.

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Sadly, one or two of the actors to have featured in It's Always Sunny over the years are no longer around, meaning that certain characters are all but certainly gone for good. For some of the others, however, it seems more like a matter of when — rather than if — they'll be coming back, particularly those who've been with the show since the very beginning.

10 Gail the Snail

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Gail serves as one of many outlets for some of the gang's more vulgar tendencies, though there's perhaps an argument to be made that they're right to "salt" her. Whether by "mashing" her uncle Frank under the table on the day of her father's funeral or through the many disgusting sounds that she makes though, she definitely deserves her title as the garbage pail cousin.

While it's certainly true that Gail can be just as annoying to viewers as she is to the gang, it's hard to deny that she hasn't contributed to some of the show's many memorable moments. These include the all-female Wade Boggs challenge in season 13 and the infamous Thanksgiving incident that lead to Mac and Dennis' apartment burning down just a few years prior.

9 Ben the Soldier

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Ben's childlike innocence only serves to highlight just how depraved the gang really is, while his lack of intelligence often leads to him being manipulated or exploited for their gain. From the moment he rolled into the show back in season 5, he's provided more than his fair share of laughs without ever really needing to be the center of attention.

Some of Ben's highlights include standing up to Dee after the guys convince her that he's been "Dennis-ing" her and foiling the gang's plans to trigger his non-existent PTSD. He's also known for his unusual attire, with a 19th-century cap and nightgown and an iconic pair of jean shorts among the unusual outfits that he's worn over the years.

8 Luther

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As evidenced by some of the many flashbacks to Mac's childhood, Luther was not a particularly good father. For starters, he named his son Ronald McDonald, but he also made Mac an accomplice to some of his many crimes. He's still a great character, however, and one that has played a prominent role in some of the show's best episodes.

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Whether attempting to convince Mac and Charlie to hide heroin up their butts or straight-up threatening to kill them, Luther makes for an incredibly intimidating villain. The fact that he never seems to blink also helps in this regard, as too does the emotionless manner in which he typically speaks. His near-constant incarceration prevents him from showing up all that often, but when he does, it tends to be good.

7 The Ponderosas

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Dennis and Dee's childhood sweethearts are initially presented as two incredibly well-adjusted individuals. After reconnecting with the Reynolds twins, however, their lives quickly begin to spiral out of control, with Bill becoming a junkie and Maureen becoming a cat (or at least, a person who identifies as one, anyway).

The Ponderosas' very best moments arguably came in season 8 when Maureen was due to marry Liam McPoyle. Dennis gatecrashes the ceremony to ensure that the nuptials go ahead as planned, but his weakness ends up costing him dearly. Bill provides the proverbial icing on the cake, which, in this case, ends up being Bath Salts in milk.

6 The Maniac

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The Maniac was a little reminiscent of Micky Rourke's character in the 2008 movie, The Wrestler. Though a lot of this was down to the two men sharing a profession, the manner in which they lived their lives was also quite similar, with wrestling having a huge impact on both their personal relationships and their physical and mental health. Of course, It's Always Sunny took these ideas a few steps further.

One of the things that made the Maniac such a great character was how little backstory the show's writers chose to give him. He was mysterious, but also incredibly unpredictable, which created the impression that just about anything could have happened while he was on screen. Many of his finest moments came in and around the ring, but his berry-selling exploits were also top-notch.

5 Uncle Jack

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Uncle Jack is one of the more unusual characters in It's Always Sunny, which is really saying something given the main cast. His strange obsession with hands and the size of his own has led to more than a few laughs, while his incompetence as a lawyer has done the same while also allowing other characters to shine.

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There's a lot of mystery surrounding Jack's past relationship with Charlie and what he may or may not have done while the two shared a room. It's heavily implied that Jack was in fact "The Nightman" though, a recurring character in some of Charlie's more disturbing songs. His love of "tasteful photography" seems to support this idea too, as mentioned in season 9.

4 The Lawyer

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He may not be all that talented in reality, but Jack makes the Lawyer seem like a legal mastermind, particularly during Dennis and Maureen's divorce proceedings. This was far from the first time that the character got one over on the gang though, with the Lawyer having also tricked Charlie into giving away the rights to his Kitten Mittens idea back in season 5.

Despite his legal superiority, however, the Lawyer actually lost the only case against the gang that ever went to a real court. That's not all that he loses in that courtroom either, with Pappy McPoyle's Poconos Swallow seemingly gouging out his right eye after Charlie's "bird-law" defense turns out to be a good one.

3 The Waitress

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Like most of his antics, Charlie's obsession with the waitress is incredibly unhealthy and would likely lead to jail time if replicated in the real world. Despite this, however, the show manages to frame Charlie's stalking in a way that's funny, by making it crystal clear that the Waitress really isn't a good person herself (particularly in later seasons of the show).

As a result of their toxic non-relationship, most of the Waitress' best scenes are shared with Charlie, which is quite fitting considering the actors who play the pair are actually married in real life. Their time together at the Jersey Shore was perhaps the most wholesome of their on-screen exploits, while Charlie's musical proposal was undoubtedly the most tragic.

2 The McPoyles

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Though Liam and Ryan are the most commonly recurring members of the McPoyle tribe, they're just two of the many idiotic inbreds to feature throughout the series. These include Margaret the mute, the monstrously large aspiring football player Doyle, and the bird-loving oddball, Pappy McPoyle.

The McPoyle family has had some wonderful moments over the years, but one of their best came in season three when Liam, Ryan, and Margarett invaded Paddy's and held the gang hostage. Despite squashing the beef on numerous occasions, the gang is constantly ruffling the McPoyle family's feathers and will no doubt continue to do so in future episodes.

1 Cricket

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When he was first introduced to the show back in season 1, Matthew Mara was a well-respected man of God. After being reunited with Dee and the rest of the gang for five minutes, however, he quickly finds himself unemployed, homeless, and, eventually, addicted to drugs. Unfortunately, as the years have passed, things have only gotten worse for old Rickety Cricket.

The street urchin has featured in countless storylines over the show's fifteen seasons, to the point where it's incredibly difficult to single out just one or two. His time as a drug dealer arguably ranks among the funniest though, as too does the season 4 episode in which Mac and Dennis decide that they'd like to try hunting humans.

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