Mikuru Asahina from The Melancholy of Haruhi Suzumiya. Haruhi is constantly trying to invoke tropes, and uses Mikuru for her appeal and tries to make her the Innocent Fanservice Girl, much to Mikuru's despair. While she eventually accepts her Meido outfit, she is still struggling to accept it when Haruhi tries to put more revealing outfits on her.
This is the entire concept of Maicchingu Machiko Sensei. The title character continually finds herself naked due to the actions of her perverted students, the occassional non-human, or the nature of the universe itself.
Takane D. Goodman is the most notorious. Takane is in the habit of using magically-generated clothing instead of actual clothing, and so her outfits tend to vanish completely whenever she's defeated in battle... and she's not exactly the most powerful side-character.
This is also known to happen to Asuna. Early incidents involved Negi's wind magic activating by accident, which blew off all Asuna's clothes without causing her any harm. Negi's wind magic accidentally activating has done that to many girls, not just Asuna. The fact that his attempt to erase her memory also resulted in nothing more than clothing loss for Asuna, however, was foreshadowing of Asuna's magic cancel abilities. And later, her magic canceling abilities became so strong that she kept her clothes on when Negi's infamous strip-sneeze came around.
This culminates in an amusing scene in the movie where Lucy runs out of the shower without realizing that her towel didn't quite catch and ends up in front of the now wide-eyed Gray and Natsu, prompting the following exchange:
Natsu: Gray, is your stripping habit contagious?
Gray: Thank you stripping habit!
Mio Akiyama from K-On!. She's been forced into her share of costumes, not to mention tripping over a cord and landing in a panty-exposing position during the band's first performance in front of the whole school.
Yuki from Fruits Basket, especially when he's forced to wear a beautiful girl's kimono for the school festival.
Yu-Gi-Oh!: In the virtual world mini-arc, Joey is forced to wear the garb of a legendary warrior: a one-shouldered "barbarian" style garment cut very short. It's even skimpier than Mai' costume — although she's extremely comfortable in hers and Joey tries to avoid coming out in his.
Ryuko from Kill la Kill, in the first few episodes at least, is extremely embarrassed about how Stripperific her Kamui is. This actually turns into a plot point: because she's so ashamed of its skimpiness, she's unable to harness its true potential until a (totally bizarre) Rousing Speech from Mako.
In the Hentai Dragon Pink, Pink is constantly being humiliated in this way.
The condition for Ringo's jumping power in Banana no Nana is that she has to wear a miniskirt that shows off her panties.
Elizabeth of the series Nanatsu No Taizai she always end up being in a fanservice against her will but doesn't seem to fully mind it.
The Magical Girl outfit in Oku-sama wa Mahou Shoujo is designed for your standard, young girl. Ureshiko, at 26, has a fully adult body, and she's not quite comfortable when people point out how it looks on her.
Karina "Blue Rose" Lyle in Tiger & Bunny is contractually obligated to go crime fighting in a Stripperiffic outfit, despite how impractical and dangerous it is. Mainly because her sponsors market her largely on sex appeal. Since she's a deconstruction of Ms. Fanservice, poor Karina is shown to be extremely embarrassed — but she can't do anything, since it's either that or losing her fans and her sponsors.
In the (banned) Pokémon episode "Beauty and the Beach", Misty declares that parading in front of strangers in a bikini is "Totally embarassing and degrading... but we need the money."
Bulma takes a turn of this in the original Dragon Ball. In the very first story arc, her clothing becomes damaged and, stuck out in the desert, the only piece of clothing nearby is a Playboy bunny costume. She hates wearing it, but inexplicably wears the bunny ears with it until she can replace it with some nearly-as-revealing local attire.
In Dorothy of Oz, Mara Shin's Magical Girl transformation strips off her clothes to reform them into the Witch's suit. Unfortunately for Mara, someone always walks in just as the magic takes her clothes off. In the fifth volume, she trips and falls on top of one of her comrades while in the middle of her transformation.
Nico Robin from One Piece is a meta example: She doesn't seem to mind wearing Absolute Cleavage and other skimpy clothes in-universe, but one fan pointed out that he found it weird that she wouldn't dress more modestly with the kind of personality she has (not only modest, but also relatively sophisticated and mature). Oda replied that indeed, a women with Robin's personality wouldn't realistically want to dress like that, but since it is a way to provide Fanservice, he will continue drawing her wearing those kinds of clothes.
While they're out sailing in episode 15, Louie's fishing hook snag's the top of her bikini. So when he casts the line, it pulls her across the deck and tears off her top, ending with her being flush against his back. Predictably, she has a screaming fit and demands her top back (at 6:48-7:12).
And in episode 21, she volunteers to cover for Merrill, while she was out sick, not knowing that one of her jobs was dancing in a burlesque show. So Melissa is equally horrified and mortified when she sees the "costume" she has to wear.
Melissa: (thinking)"OH MY GOD!! If this is what he means when he says "costume", I don't want to know what he means when he says "dance"!"
The entire point of Empowered, where the titular heroine's super-suit seems to be perversely designed to leave her as exposed as possible and tear to shreds at the drop of a hat. This has not helped her self-esteem any.
Played with with the New 52 Power Girl, seeing as she's always getting into situations where she might be invulnerable, but her costume clearly isn't. It ends up getting torn pretty much in every episode, to her dismay. Then when (Earth-1) Supergirl's Sanctuary gave her the old Power Girl costume (complete with infamous boob window), presumably now invulnerable like Supergirl's, as a replacement, she seemed quite happy with it. Clearly it's not the style of costume that's the problem, just she'd rather have one that stayed intact.
DC villain Plastique became this at the end of her first appearance when Firestorm used his powers to de-atomize her clothes, leaving her completely naked and sexually humiliated in public while he took away the bombs that were attached to her suit.
Jane from the Daily Mirror strip of the same name. An urban legend claims that during World War II, when a rumor went out that D-Day would coincide with Jane appearing fully naked for the first time, army brass specifically asked the artist to make it so.
Films — Live-Action
A few of the Bond girls across the James Bond franchise actually fall into this, but most tend to be very knowing and savvy about their sex appeal. Interestingly, Plenty O'Toole goes from being a rather Shameless Fanservice Girl to a very reluctant one once she gets caught stripped down to her panties by the "Pervert's Convention" in Diamonds Are Forever
Sandy in Saving Silverman has been in training to be a nun and wears a knee-length habit. When she needs a change of clothing, the only outfit available is a set of stripper clothes.
Mercy Thompson is frequently naked, as when she changes shape, her clothes don't come with her. Played with, in that by itself she doesn't mind being nude at all. It's only that people make assumptions when she shows no qualms getting naked around them, and she wants to avoid such unnecessary assumptions.
Amelia Windrose from Orphans of Chaos, constantly. A climactic scene in the third book is enacted with her naked the entire time, and a villain standing between her and the only towel. It's... much more serious than it sounds.
Widespread, and applied to both genders, in Spartacus: Blood and Sand. Slaves are often forced to go partially or completely naked for the entertainment of their Roman masters either for erotic reasons, especially with household slaves and prostitutes, or simply because in some jobs, such as mining, they see no reason to provide them with clothes.
Happened to both Xena and Gabrielle a few times on Xena: Warrior Princess, one of the most famous examples being The Bitter Suite where Ares gave Xena one epic Shameful Strip.
Natalie in Epic Battle Fantasy, after using her Kyun Limit Break, will usually comment on it afterward by saying, "Well, that was embarrassing", "I hope that's enough fanservice for a while", and "Okay, stop staring now."
Mikan Tsumiki in Super Dangan Ronpa 2 is sometimes an over-the-top parody of this, due to how she'll often end up in extremely embarrassing poses (though any Panty Shots she might be displaying will be conveniently censored in some way). However, she's also somewhat of a deconstruction since being the Reluctant Fanservice Girl only makes her already low self-esteem even lower, and yet she's painfully used to this being the only way she can get any sort of attention from people.
Olivia from Fire Emblem Awakening is a dancer who dresses rather skimpily... Yet she's also a Shrinking Violet who suffers from Stage Fright when off the battlefield and who refuses to wear a swimsuit in the Summer Scramble DLC stage. She explains it in some support conversations as not really thinking about it during battle, since she knows that her dances are necessary for letting other units move twice.
Surprisingly, Tharja is one of these. In the same Summer Scramble DLC stage, she's given a skimpy swimsuit to wear and is not exactly pleased, and when Cordelia talks to her in said scene she gets very upset when Cordelia keeps bringing up her figure during their talk. Also, her usual portrait has her looking extremely awkward and using her spell book to cover herself (Contrasting with the also well-endowed Aversa, who veryproudly flaunts her Absolute Cleavage) Like in Olivia's case, Tharja's super skimpy outfit is a necessity of sorts, since it's the average uniform for female Dark Mages, and it's implied that she underestimated its effect on those around her.
The Princess in A Dance with Rogues is this, if she isn't played as a Shameless Fanservice Girl, since almost all of the module's female clothing is revealing and she's frequently put into situations that force her to infiltrate or sneak around naked, or lose her equipment in escape attempts. She canlampshade this by showing her exasperation with the ridiculously impractical design of the clothes and being annoyed at the lack of equipment in several quests.
Grant Kirkhope was allegedly this when he was a guest on Game Grumps; according to him, the Grumps do their show naked, which, with everyone beingThe Unseen, took him by surprise and made him uncomfortable. When Jon mentions that he didn't have to join them in their nakedness, Grant responds that he'd be even MORE uncomfortable if he joined them clothed, but is still upset to "let his dingleberries out."
Girl: Hercules, the Cyclops tore off my clothes! Hercules: Ha-ha-ha-ha-ha!
Marge herself becomes this a few times, most notably in Natural Born Kissers where she winds up trapped outside completely naked after having sex with Homer at a miniature golf course. She is then forced to run all over town completely naked with the equally naked Homer by her side in the hopes of getting home before they get seen without their clothes. This does not end well for them.