Ichigo Kurosaki, whose street clothes early on always seem to expose his belly if he does so much as lift his arms.
Female examples include the Arrancar Halibel, Neliel, Mila-Rose, Lilynette, Loly, and the Visored Lisa, in whose case it's a Sailor Fuku many times too small.
Inoue's outfit when she and Chad meet up with Ichigo in Soul Society in the Thousand Year Blood War arc has this as well as Cleavage Window, though it's really just very wide straps. (You can see her skin on her side in color pages, and Urahara even says, "If you close it in the front it'll spill out the sides!")
This outfit◊ worn by Sylia Stingray in the Bubblegum Crisis OVAs, sometimes with a suit jacket over it. The first episode was released in 1987, but with the passage of time her fashion starts looking less and less ridiculous.
In Endride, this is a staple of men's fashion in Endora. Among others, Emilio, Delzaine, Guidoro, and Gradido all show off their abs with either open fronted-shirts or unusual cutouts. It may be a case of Author Appeal as, of the two character designers, the ones done by Kazushi Hagiwara feature this most often.
In Eureka Seven, Gidget, Hilda and Talho all have outfits that bare their midriffs to various degrees. Talho, however, switches to a full-body costume later on.
Hellsing's Doktor, who is also a Nazi. What is most disturbing to fans is that he pulls it off remarkably well.
Seras also did this in the fourth episode of the TV series.
Both Hinowa and Tobari from Hinowa ga Crush! are always seen with their respective midriff baring outfits.
In If Her Flag Breaks, Megumu's swimsuit in episode 6 has a top and bottom portion. Though he doesn't really need it as a guy, it's played up as if he were a girl, and Nanami hilariously does an Eye Poke on Souta when Megumu first takes his shirt off.
Joseph Joestar, the protagonist of Part 2. A particularly notable case in that he actually predates the Bishōnen Jump Syndrome phenomenon by quite a few years, though it doesn't come off as any less fanservicey.
For the usual (female) example, there's Jolyne Cujoh, the protagonist of part 6.
Weda from Haré+Guu wears a similar outfit to Lum's. Villagers Rebecca and Addy wear midriff-baring outfits as well. It's played with by Guu herself. She always wears the same long pink skirt and yellow tube top, but it only shows off her midriff in her adult-form.
Many of the characters in Kaleido Star (including some of the male characters) wear midriff-baring outfits as their stage clothes, casual clothes, and training clothes. Especially Anna Heart, and Rosetta Passel.
A cameo of Misaki who appeared in the Episode 10 of an anime Kobato. wore a midriff-baring blue promotional outfit during the end of an exam along with Tamayo. This is the first appearance of Misaki Suzuhara who bares her midriff in the other CLAMP anime.
Lampshaded in KoihimeMusou, by Aisha, Rinrin and even Shuri, commenting that almost all the generals from the Wu kingdom are showing their bellies.
In K-On!, the former Light Music Club band Death Devil used to have outfits with this. Also, one of the bands at the Live House show had this in their outfits.
The main girls rarely expose their bellies outside of wearing bikinis, but Ritsu wears a hula outfit in her Imagine Spot about going to Hawaii, and wears a midriff-baring shirt when going to apply for passports. In addition, Azusa has only worn bikinis in merchandise, preferring one-pieces otherwise.
Raichou's Magical Girl costume in Kyouran Kazoku Nikki has a big gap, through which she shows off her stomach. In one episode she dresses up Kyouka in a costume that reveals even more in that department.
In A Little Snow Fairy Sugar, the woman who plays the bear pianist's human form in the play "The Bear Pianist" wears a midriff-baring tank top normally.
Yue's outfit bares her midriff after the fight in the Governor's Ball. Reminding you that her ally was written out of existence by a shot that passed through Yue, destroying that part of her shirt along with Emily.
The girls of Hidamari Sketch rarely wear outfits that do this outside the context of swimwear, but one time, when they were taking pictures of each other with Sae's new used Polaroid camera, Hiro lent Yuno an outfit that showed her tummy. Sae was surprised she even had something like that (with a skull and crossbones on it), but Miyako guesses that Hiro wanted to be bold, so she bought it, then decided she couldn't wear it.
Yoshinoya-sensei wore a midriff-baring outfit on stage during the opening ceremony in Volume 4, and later to her class, helping her illustrate the three elective enrichment classes available to second-year students. She invited the ones who had already decided to touch the part of her that indicated the class they wanted: her tummy for 2-D, her breasts for 3-D (!), and her head for information.
While the girls of Strawberry Marshmallow don't seem to wear these in the main story outside of swimsuits, Nobue and Miu do on some episode pages and interstitial pages.
Misaki Suzuhara did not bare her midriff in the anime, but she bared with her another swimsuit in the official anime picture that shows a group picture of her, Tamayo, Hatoko, Sai, and Kaede in their swimsuits.
Amalla Su from Love Hina: Her default outfit is rather revealing tribal garb. Kaolla Su (and later the rest of the girls) wears a similar outfit for her wedding dress.
The known mass-murderer Kagami Naoto wears a midriff-baring outfit in an act of Fan Disservice, considering how emaciated he looks.
Kumo Kagemitsu, one of the Kumo brothers' ancestors, technically wears the attire of a shinto priest, but it's cut so short that it reveals his midsection. Combined with his long hair and soft facial features it leads to a Dude Looks Like a Lady moment when Hirari first meets him.
Gengetsu's stripperiffic ninja outfit combines this with underboobs. Although she is also an expensive prostitute and knowledge broker.
Somewhat lampshaded in Magic Knight Rayearth: The ship from Chizeta is full of young women wearing such an outfit. When Umi is captured, since her clothes disappear, she's given the same kind of outfit, which makes her complain about her bare tummy.
Ino shows her midriff starting in Shippuden and continuing through adulthood. Prior to Shippuden she subverted the trope, wearing a separate top and shirt that left her midriff exposed... but covered with bandages.
Sai bares his midriff in Shippuden and The Last-era episodes, but not in adulthood.
Karin is a partial example: her lab coat hangs all the way down on the sides, but is cut to reveal her belly in the front and back.
Fu, the female Seven Tails host.
Sakura bares hers in the entirety of the third movie, Guardians of the Crescent Moon Kingdom.
Her currrent default outfit in Boruto has her baring her midriff.
Shion from Naruto Shippuden: The Movie.
She partially bares her midriff during adulthood in a similar fashion to Karin.
Momoko Asuka, though it's a bit risqué for a fifth grader.
During one episode of Dokkaan, Hana-chan dons a costume that also has this.
One Piece: Almost all of the female characters in get in on this trope.
Nami, and to a certain degree Robin, are prime examples.
Even Dr. Kureha (complete with bellybutton piercing), despite living on a polar island and being 139.
Pokémon pretty much takes this trope and runs with it:
Serena has demonstrated this multiple times: During the third anime outro, in her Fennekin outfit◊, and in her bathing suit.
Misty wears a midriff-baring top in the anime, but shows a lot more skin in one of the manga series, donning a higher-cut shirt and lower-cut shorts. After Misty was Put On The Bus she returned with a new outft that covers up more.
Jessie's outfit is this way on default.
Tyra from The Bridge Bike Gang of the Original Series.
Duplica, from Ditto's Mysterious Mansion, when she wears a Team Rocket outfit.
Liza, the caretaker of the Charicific Valley's outfit.
Ahiru in Princess Tutu wears a midriff-exposing sweater when she's not in her school uniform — although on her it has more of a silly effect than a sexy one. The Dark Magical Girl, Kraehe, also has a dress that covers her chest.
Early concept art which had the original◊ five◊ with more individual costumes. Ami Mizuno, Minako Aino, Rei Hino, Makoto Kino, Hotaru Tomoe, and Michiru Kaiou with shirts that stopped just short of the belts.
Kirara in Samurai 7. As well as a most of the women from their village.
In Tsubasa -RESERVoir CHRoNiCLE-, although Fai's default outfit doesn't include this, he shows his belly multiple times in his Hanshin, Piffle, and Infinity World outfits. Sakura's default outfit, as the princess of a desert country, applies too.
Some of the horse girls from Uma Musume wear this as racing outfit, but for the Winter Dream Tournament on the last episode, all participants wear special, midriff-baring outfits.
When Carly Carmine becomes a Dark Signer, her outfit consists of a black midriff top with a skirt and a cape to boot.
That Jerkass Barbara that Yusei meets in Crash Town.
Kalin Kessler as a Dark Signer to a lesser extent.
Yumekui Merry: Merry. Notable in that they're not trying to "sneak" anything past the radar—in fact, the animators appear to be actively trying to put in as many shots of Merry's bellybutton as they can.
Juri from YuYu Hakusho wears a one-piece outfit with a large section around her belly cut out.