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.
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.
Blue Rose 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.
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.
The entire point of Empowered. This has not helped the title character's 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.
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.
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.
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. 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.
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!
Fluttershy ends up as one in the My Little Pony: Friendship Is Magic episode "Green Isn't Your Color" after Rarity talks her into being a model for Photo Finish. She hates being a model but keeps at it because she doesn't want to disappoint her friend. She ends up quitting at the end of the episode after Rarity finds out how miserable she was.