You're certain to find Tsundere characters in fan fiction, since most of them are written by a bunch of perverted otaku who use it to act out their sick fantasies with their creepy fan-crushes. And by that, I meant you, idiot!
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When dealing with romances between a protagonist and an attractive antagonist in fan fictions the difficulty of the antagonist having a highly abrasive personality often arises. Transforming the antagonist into a tsundere and setting their attraction on the protagonist allows for character development and easier movement away from "evil".
It's also common for characters who hate each other and/or are sworn enemies in canon to be portrayed as this by fanfic writers who want to hook them up together but also want to make the pairing at least marginally more plausible than the alternative of completely ignoring their canon hatred of each other, portraying them as only thinking or acting like they hate each other because they can't/won't admit that they actually love each other. Also see Foe Yay and Belligerent Sexual Tension.
ThisMetalocalypseSlash Fic explores, among other things, how William Murderface's canonical Type A Tsundere tendencies might express themselves in a romantic relationship.
Murderface was strange. Toki knew. He was unstable, and needy, and seeking approval at all times, while at the same time hating himself for everything that he did. He couldn’t exist happily on his own, even though that was what he longed to do. Half of the time, he couldn’t even understand what it was that he wanted. He hurt himself out of frustration. He tried to hurt others out of anger. But he couldn’t hurt Toki. Not anymore. Not when he quietly invited Toki in to sit on his bed. Or when they stared at each other at dinner. Or when Toki softened him, slowly, gently, and they lay together in bed, asleep or awake, alone. Then, Toki was invincible. Then, Toki could brave the thorns around that tiny, warm heart that beat at the same pace as any other human being’s. That tiny heart that Murderface had wanted to give to him, because he was the only other person that had ever managed to touch it.
Zara of Toy Hammer insists—quite loudly—that she's not one of these. "I am not a hormonal teenage girl with a crush she can't admit!" (From the looks of it, the only part of that sentence that's true is that she's not a teenager.)
Tatl Beryllia in The Blue Blur of Termina. She regularly switches between belitting and snarking at him, and being surprisingly cool with him, to the point of lightly flirting with him at times.
The Dragonborn in the Cross Over fic, Portalborn is a prime example. She's mostly tsuntsun to nearly everyone and sometimes deredere with Farkas. The conflict she has with Doug Rattmann is more tsun and rarely dere, but he's normally just too scared to do anything about it anyway. He gets a little bit braver in more recent chapters and is stated by some characters to be in love with her. Judging by the Dragonborn's confused thoughts, it's probably mutual.
If a character is naturally shy in the original medium, it can be difficult to bring them into the spotlight in a fan-work. As a result, authors often give them buried tsun-tsun characteristics that emerge in response to a key point, giving the player a backbone and respect from others.
In the crossover fic Action Pack Naruto himself is portrayed as a Type B, while his brother is in a whole other category...
While Greece is normally portrayed as a sweet, laidback guy who's nice to everyone (especially to Japan, his most common love interest) and acts differently around Turkey only because of their long rivalry (and Turkey's interest in Japan just adding more fuel to the fire), it isn't too hard to see how that interpretation can be easily rewritten to make Greece into a Type B tsundere who's friendly to everyone except the one he's actually secretly in love with. Some Turkey/Greece fanworks even essentially do away with Greece's 'default' Nice Guy state (or simply show him interacting only with Turkey and thus not having much of a chance to show his 'dere' side with others), turning him into an all-out Type A tsundere. This can lead to quite a bit of Mood Whiplash while reading Greece fanfics, going from Greece/Japan fics where he doesn't display the faintest sign of tsun to Turkey/Greece fics where he goes violently tsun with physical blows and insults - yes, even the shipping itself is tsundere!
Some fans see Japan as a Type B tsundere toward his former adoptive brother China, as China appears to be the only nation Japan openly expresses annoyance or irritation around. Though considering that Japan once stabbed China In the Back, some other fans see Japan as fitting a different trope more in regards to him...
Heck, even a few Greece/Japan fanworks depict Japan as a partial Type B Tsundere toward Greece, getting along with him excellently until Greece goes into Chivalrous Pervert mode which leads to Mr. Spazzes-Out-At-A-Mere-Kiss-On-The-Cheekblushing furiously and denying just as furiously that he wants anything physical or outright shouting at Greece or shoving him away for daring to bring that topic up. (Good thing Greece is so persistent/patient with him, huh?) One doujinshi even had Japan deliver Amusing Injuries on Greece and give him the cold shoulder for a while after they ended up making out in an RPG (don't ask).
China himself is sometimes depicted as a Type B tsundere in Russia/China fanworks, usually being implied to have become one out of necessity of withstanding Russia's childishly sadistic side without breaking or showing vulnerability.
Also happens in most China/South Korea fics, with China's outward annoyance toward Korea's antics being a cover for his deeper-seated feelings for him.
Taiwan becomes a Tsundere when she's paired up either with the aforementioned Korea or Hong Kong.
When not shown as an Comically SeriousSugar And Ice Guy, HK is often written/drawn as a Tsundere as well. Specially with Taiwan and Korea, too: he insults and snarks at them with a perfectly straight face, then blushes and mutters compliments when no one but Taiwan and/or Korea can hear him.
Also, when Hungary is paired up with either Turkey or Prussia instead of Austria, she becomes a pretty snippy and violent Tsundere towards them. Taken Up to Eleven if her beau is actually Romania.
The British Isles (England, Scotland, Wales, etc.) tend to be written as an entire family of Tsunderes, to varying degrees.
The already mentioned Iruka Umino of Naruto is often portrayed in a Type B light when paired with Kakashi. (This seems to have something to do with his canonical tendency to blush easily and wicked temper.)
Beth Lestrade, in her original show, fits Type A quite well. However, in Children of Time, she is solidly Type B — justified in that she's several years younger than she is in the show, and she's not done yet developing as a person. She does her best to be a good friend to Sherlock Holmes, very much in love with him but convinced that he'll never return her feelings. However, after a day of very thorough abuse from him (betrayal of trust, handcuffs, insults), she starts lobbing insults back with precision and ends up slapping him when he crosses the line. She regrets it very quickly and kisses him on the abused cheek, and accepts his apology a few minutes later. She eventually earns her happy ending in the most traumatic way possible.