In Eleutherophobia, Tom got into the habit of swearing mentally to annoy Temrash 114. He never broke it, so he sometimes forgets to watch his mouth while talking to children and/or relatives.
In What Tomorrow Brings, Tom spends most of his first chapter mentally swearing at Temrash.
In Treefall and its sequel Sunrise, although almost everyone tends to use swear words when they are in the face of conflicts (including the teachers who are supposed to be role models for young generation!), no one quite compares to Frenda Seivelun and Misaka Mikoto in terms of vulgarity.
Crossovers
Amazing Fantasy has both Bakugou and the Prowler. The former is known for his Hair-Trigger Temper and ends up Flipping the Bird at Mysterio while continuing to mock him for his obsession with killing Peter. The latter, during her fight with Peter, she also curses up a storm out of frustration, but Peter couldn't understand a lick of it at the time.Given that Prowler is actually Mitsuki Bakugou, it's clearly In the Blood.
Nick Fury and Peter Wisdom, both grumpy Good is Not Nice spymasters (and in the latter case really Regulus Black, reformed Death Eater), frequently swear.
Sean Cassidy's cuss words aren't usually elaborated upon, but they tend to make people's hair stand on end - and on one occasion, after he's narrowly escaped being sucked into a tornado (It Makes Sense in Context) - Thor remarks that he's never heard a human swear that much and that creatively. Sirius and Tony take it as a challenge. Pepper informs them that they will present a good example to Harry, won't they?
When aggravated, Wanda apparently has 'a vast repertoire of obscenities'. Having previously dated John Constantine, it is not hard to imagine where she learned many of them.
Carol's pretty free with dropping f-bombs, probably mostly to annoy her father, and was taught them by her grandmother and her Cool Uncle, Jack O'Neill (who complained that his mother had already taught her all the good ones). This is rendered Hilarious in Hindsight by the reveal that she's the great-granddaughter of Captain Steve 'Language!' Rogers, who she later goes on a tirade against in English, French, Spanish, Arabic, Irish, and Russian. As Harry (who has a moderately foul mouth himself) notes proudly, he taught her most of the Russian ones.
Speaking of Harry, he's got a relatively foul mouth when the situation merits it, and can swear in fluent Russian in the sequel. A Running Gag in the Mirror Image arc of the sequel is him doing his best not to swear around Clark and his parents.
In Don't Lose Your Heart, Ryuko is just as foul-mouthed as she was in Kill la Kill, though she gets to go a bit further with multiple f-bombs. The author even warns readers ahead of time that she's got one hell of a mouth on her. Even years later as a Keyblade Master, she's still as vulgar as ever, something Ira lampshades.
Ira: One cannot question the strength of your heart. You're a Keyblade Master. Your authority surpasses even that of kings and regents. But must you speak with such vulgarity and uncouth language?
In Incarnation of Legends, Despite her gentle, serene public persona, Brunhilde's true personality is coarse and gritty, swearing up a storm wherever she goes.
In Origins, a Mass Effect/Star Wars/Borderlands/HaloMassive Multiplayer Crossover, Jack is noted to be this, but, as in canon she has to keep it down around her students, since there is an actual Swear Jar. To further lampshade her status, most of the other characters keep their cussing to a minimum.
Dante's vocabulary in Son of Sparda D×D is more in line with how it was in Devil May Cry: The Animated Series, resulting in him having a very vulgar tongue. He even calls Riser "pussy-splashing thundercunt" among other things.
Souls Reborn: Ginta, who was taught how to talk by sailors. The ironic part? He's Gaara's reincarnation.
Danganronpa: Paradise Lost: Shun is noticeably far more prone to swearing compared to the other students. An example comes in his response to Monokuma informing the students that they are trapped in the facility: while the others do protest, Shun is the only one to use profanity, calling Monokuma a "son of a bitch".
In Forest Of Despair, both Akita and Mitsuru talk like sailors compared to the rest of the cast. This is even lampshaded by Akita herself.
The Angel of the Bat trilogy features plenty of dirty-mouthed characters, but John Constantine is the most vulgar of the bunch. Tellingly, religion-based curses are usually treated as particularly taboo in the trilogy, but Constantine ends up shouting, "Jesus Christ!" in the story's first chapter.
Cor Et Cerebrum: Kiran "Dev" Devabhaktuni is an exceptionally foul mouthed surgeon, though he does his level best to cut it out around underage patients and his nieces and nephews.
In A Brighter Dark, Hans, a lowborn warrior and career criminal, has a crude manner of speaking that includes plenty of F-bombs. For the first half of the story, Corrin leans heavily into this when talking to or about her Hoshidan enemies.
In An Eagle Among Lions, Joachim, one of Edelgard's older brothers, swears far more often than any of his siblings and is frequently scolded for it. Even Jeralt, who himself is known to have a foul mouth, is surprised.
Cristina from The Cut of Your Love Never Hurts, Baby spends a good chunk of her dialogue cursing. It's especially noticeable because no one else curses.
In The Lost Cause, Nie Huaisang is this in private when his mask is off. In public, he keeps to a more appropriate vocabulary for a young lord of a major sect.
In the A Troublesome Charge series, A-Qing is this. It's how Xue Yang recognizes her when she is possessing Song Lan.
My Immortal: No one other than Ebony Dark'ness Dementia Raven Way.
Enoby: What the fuck? You torture my bf and then you expect me to fuck you? God, you are so fucked up you fucking bastard.
Veronica "Ronnie" Weasley, a Gender Flipped version of Ron Weasley from the fic Weasley Girl is about as fond of swearing as her canon male counterpart, but gets censored a lot less and is allowed to drop a few F-bombs:
"I'LL KILL THAT FUCKING BASTARD!"Said about Voldemort/Quirrell, after he's placed her under the Imperius curse and Harry manages to break her out of it.
Kairi is portrayed as this, which she justifies with the fact that she grew up in a port town, though Riku is rarely shown swearing and Sora is only known to have done so once between chapters.
Vanitas also stands out, since he's the only character able to swear around Sora (there's a partially self-imposed Narrative Profanity Filter around him which even Kairi is happy to adhere to).
In One for All and Eight for the Ninth, Ochako proves she's a construction worker's daughter by swearing up a "creative and disturbing" storm when she sees the "casual eatery" Momo takes the class to after their first day. She repeats the feat before her match with Bakugo - with such an intense Cluster F-Bomb that Bakugo is left dumbfounded.
The Darker Sides of Deku: Katsuki, as usual, but in a twist it was Izuku's mother who taught him. He never looks as proud as when he explains how Inko Midoriya taught him how to swear. Izuku gets into a minor argument with him, insisting that he's missing the first lesson about only swearing when you really mean it. Katsuki insists he always means it, and besides, he prefers Inko's second lesson—chain swears. Katsuki and Izuku get into a swear-off that lasts the entire class period.
In Perfection Is Overrated, Starter Villain Hitomi Kirihara swears significantly more than the other characters to reflect her abrasive personality and refusal to comply with social norms; she's essentially impossible to get along with when she's talking with other people instead of killing them. On the heroes' side, Natsuki and Nao also have a fair amount of profanity compared to the rest of the cast.
A Diplomatic Visit: Chapter 8 of the sequel Diplomat at Large notes that Squirk, the monstrous octopus who once ruled what is now Aquastria, has quite the mouth on him, though readers are spared any examples. His prison is specifically enchanted so the nearby residents don't have to listen to his words.
The main character of The Twilight Child is mentioned as swearing a lot, but the Narrative Profanity Filter and Hold Your Hippogriffs usually stop her from saying anything truly offensive, though she does get in a "bastard" at one point. However, when she's truly at the end of her rope, she prefers to unleash the foulest word she can think of: "Belgium."
Bubbles, of all characters, is this way in the The Powerpuff Girls fanfic series Ladder. Despite being normally innocent, she curses quite a sum, especially when she goes into her seizure-like states. It's very uncharacteristic of her, so Professor Utonium thinks it's a result of brain damage caused by bringing her back to life.
Fanon in general often portrays Aran Ryan as being very foul-mouthed, but for specifics:
He drops several swears in Ask the Ryans, as does his sister Rory (though, being a child, Rory doesn't use as strong words as Aran).
At least one Ask the WVBA post has him say "feck"note not a typo, but a common Irish slang word, considered a softer version of the other word at least five times.
In Prelude to Chorus, Felix provides almost all of the profanity in the fic. Only four swear words are uttered by people other than him. Even his internal monologues are littered with profanity. He still swears a lot in the sequel, Soul Clef XI, but there are more characters providing profanity other than him.
Melanie Black, Mercury's mother, from The Black Hearts. Not only does she provide most of the profanity in the fic, but she also ends up being the most foul-mouthed character in the series she comes from, The Makings of Team CRME. She's the only one liberally using f-bombs whereas only a handful of other characters actually even use one.note Surprisingly, it is Cinder of all people who uses two in the whole series. Even her short appearance in CRME had her swearing up a storm even though she didn't use any f-bombs then. Later on, Emerald makes an illusion of her and just about every sentence she utters has a swear word in it.
Mom always had a way with words.- Mercury's narration in The Black Hearts
In Saki: After Story, Teru and Sumire do most of the swearing, with Teru having anger issues, and Sumire calling Teru's claims that Saki is not her sister "bullshit".
Cursing is common in Prison Island Break, but Shadow is particularly coarse, to the point that he makes "fuck" cease to have any more meaning than "said".
In the Star Trek Online fic-verse that started with Bait and Switch, the main viewpoint character Captain Kanril Eleya swears like a sailor, appropriately enough. Rarely does a chapter pass without a "phekk" or "sher hahr kosst". Other characters are more restrained, though there's still the occasional Precision F-Strike.
and its sequel Sunrise