Some characters just happen to be bisexual. They aren't sociopaths or polymorphously perverse incarnations of uninhibited slutdom; they're just folks who can be attracted to people of different genders.
Bi The Way is uncommon in mainstream media (though almost a logical necessity in others) in part because a lot of both straight and gay people appear to believe that bisexuals are somehow 'cheating' by playing both sides of the field. All may be fair in love and war but there's something about the idea of losing someone to the opposing team that makes the "betrayal" worse because that's the one level where people who are exclusively straight or gay are either unwilling or unable to compete. Add in the fact that people tend to be paranoid and have low self esteem and there's a lot of hostility out there. If Bi The Way is played straight anywhere it tends to be in Biography.
"Outing" a bisexual can be convoluted. Some pass for straight or gay or allow others to make their own assumptions. Bisexuals are not portrayed so much for their mannerisms as their supposed habits. As a result, a het-seeming bisexual reads as Straight Gay, whereas a bisexual that acts Camp Gay is likely to be seen as subverting the stereotypes associated with a camp persona. Actual bisexual stereotypes are generally limited to glam rockers, hippies, swingers and other flamboyant presentations of alternative sexuality. So in the end it's likely that Bob or Alice Bisexual will be outed in passing.
Compare If It's You, It's Okay, where someone who has shown interest in a certain sex in the past has one exception. Contrast Depraved Bisexual, where liking both sexes makes you crazy (or conversely, being crazy makes you indiscriminate), and Anything That Moves, where bisexuality implies a lack of discrimination in partners. (Believe it or not, the two are not necessarily linked; just as straight and gay people don't find everyone of their preferred sex attractive, bisexuals are attracted to some men and some women.) Also contrast Suddenly Sexuality, where a character skips to the complete opposite end of the Kinsey Scale with no warning.
Compare Casual Kink, which is the same thing for BDSM and fetishism.
The most likely bisexual archetype to lead to But Not Too Bi.
Ai no Kusabi: Despite not reciprocating Mimea's feelings and being in love with Guy, Riki is suggested to also be attracted to women in the novel by masturbating to the female form. Something which even triggers Iason's jealous anger.
Axis Powers Hetalia gets a lot of this due to the sheer amount of Ho Yay and a disproportionate gender distribution, as well as the fact that practically any pairing is possible with the assistance of a History book. While the fans pair up almost every combination, the manga itself has a number of instances of this trope either directly stated or through a cellulose layer of subtext:
Grampa Rome, who on a visit with Germany, expresses regret he never got around to having sex with a boy.
Germany and Italy. Italy hits on girls all the time, but his first real love was the Holy Roman Empire. And, following the theory that HRE = Germany, then he fell in love with Chibitalia when he thought the other was female...
Lithuania is head over heels in love with Belarus despite her wanting her brother, Russia. He and Poland also shared a house for a couple hundred years, after they and their bosses got married in volume 4, with Poland demanding Lithuania to show him his penis (likely reference to Jadwiga wanting to inspect her future husband Jagiello's naked body)
Loveable Sex Maniac France has no problem going after guys (like trying to take advantage of a sick England and coming onto an underage Italy) or ladies, as seen with him flirting and heavy-petting with Seychelles. Also, the author himself even mentions that France is attracted to beautiful men and women. As well as some non-humans.
Prussia is at times hinted to like Hungary and is also shown to like Italy very much, only referring to him as "Italia-chan", considering him 'cute' and even going as far as to ask him on a date, only for Italy to be oblivious to his question, having been fixated on Prussia's bird.
Austria has been married to Spain and later to Hungary.
Turkey is known for pursuing Japan and possibly Iceland, but in the past he also pursued Greece's mother Byzantine.
Spain was married to Austria, proposed to marry Veneziano (who he lusted for in Italy's teen years to the point of making Italy creeped out) and Romano, he has a fantasy of having the Italy brothers as his personal maids, but he also has shown interest in random girls.
Hungary was once married to Austria, but in a recent strip, practically flirts with Ukraine.
Throughout the series, Shinji Ikari of Neon Genesis Evangelion shows plenty of interest in the many pretty girls he's surrounded with. It is only after Kaworu comes along in episode 24 that we find out that he wouldn't mind a sufficiently pretty boy, either.
Revy: Inprison... I've won over some sex-starved dykes by playing the man's role. And... after that, their fingers just weren't enough for them ever again.
Possibly the original Bubblegum Crisis. While the portrayal of the girls remains rather ambiguous throughout, they all make at least one or more strongly suspect comments (in fact, during the entire eight episode run, Priss is the only Knight Saber who even allows for the possibility - reluctantly - that she might actually be attracted to a guy).
Sylvie even goes so far as to admit that the sex of a potential partner isn't important to her. But then, she's a sexaroid boomer; do androids/gynoids that were designed as pleasure units even count?
We're also told that, before being recruited by Sylia for the Knight Sabers, Priss did have a boyfriend. But she seems to spend a great deal of time with - and affection on - members of the fairer sex during the series proper.
This series was probably single-handedly responsible for the stereotype that "all anime women are lesbians, unless they're bisexual."
The reboot, Bubblegum Crisis 2040 has Sylia. While she and her mechanic Nigel are Friends with Benefits and confidents, she can act pretty handsy towards the other Knight Sabers. I.e., she practically checks Linna out when they meet!
Possibly Sera Masumi from Detective Conan. She's a 17-year-old Bifauxnen who has no problem telling girls her age (like Ran) that she finds them pretty and "her type".
D.Gray-Man had this when Kanda Yuu is revealed to have been looking for a woman that he remembers loving in a past life for all his life. When he discovers that this woman was reborn as his male childhood best friend, Alma, he still chooses to run away with him without a single moment of thought to his gender. While many fans argue the feelings were strictly platonic, one must keep in mind that the prototype for Allen Walker in her debut manga was a person who was a female soul trapped in a male body and made several remarks that indicated attraction for the prorotype Lenalee. Bisexual swordsmen? Not that much of a stretch.
Lavi too. He goes nuts whenever he sees a pretty girl but he also has considerable Ho Yay towards Allen and Kanda.
Durarara!!: Mairu Orihara, though she prefers girls over boys. Her sister Kururi's heavily implied to be bisexual as well (for example, she mentions in an online chatroom that she thinks Girl on Girl Is Hot), but not openly so.
FAKE: Dee Laytener is bi according to his own word. He does tell a 12-year-old girl that he would definitely date her in ten years. But because he is attracted to Ryo, another guy, during the manga, he comes out mostly as gay.
In the sequel, FAKE II, Ryo tells a story about a girl he fell in love with when he was younger. So he counts, too.
Ayame of Fruits Basket, who is in love with his adorable assistant Mine Kuramae but has no trouble flirting with Shigure. Apparently, when he was Student Council President he claimed that if needed, he'd give himself to both men and women alike.
Haru as well. He told Tohru that Yuki was his first love within two or three hours of hitting on her.
Mangaka Natsuki Takaya is on record as having said that Shigure Sohma's editor, Mitsuru, does not know (or, apparently, care) which sex Ritsu Sohma is, despite the fact that she is dating him.
Chou Ryuuen aka Nuriko from Fushigi Yuugi, if you take his canon feelings for Hotohori and his more ambiguous thoughts on Miaka as this.
Tsugumi Rosenmeier of Genesis of Aquarion expresses an explicit infatuation with Reika, while sharing a much more playful and comfortable, but still fairly obvious attraction to Jun. Oh dear.
In the novels, Lemon seems to be Bi The Way towards Sousuke. He had a crush on Nami, especially due to her cute, flirty teasing, and is shown to be rather pervertedly happy when he accidentally found himself groping her breast while she's sleeping. But at the same time, it's shown numerous times that he has a strong attraction to Sousuke. When he first saw Sousuke, he thought Sousuke was verygood looking and "charming," despite Sousuke being completely emotionless (read: "eye candy"). Later, after Sousuke wins a tournament that helped win back Lemon's money, Lemon hugged Sousuke with "more passion than a french kiss would warrant," making Sousuke slightly uncomfortable. And later still, Lemon gets drunk, and subsequently invades Sousuke's personal space by hugging and groping him while blushing - which irritates Sousuke to no end.
In the manga, Major Motoko Kusanagi lives with her boyfriend, but also regularly enjoys some sexual VR-program with some of her female friends. However, whether or not this isn't solely because it's impossible for a woman cyborg to have sex with a male hasn't been specified.
In the anime, she has no boyfriend, but still enjoys the company of her long-term female friends - Ran, seen coming out of the shower as the Major and Kurutan are discussing various complex philosophical issues (in keeping with the series' typical expectations of its viewers).
The boyfriend from the manga makes a cameo in the second season of the series. He doesn't encounter the Major in person, but recognises the folks from Section 9 as "Motoko's people", indicating that they did know each other and maybe had a relationship in this continuum, as well. But she also has distinctly flirty dialogue with Kurutan in another episode of the first season, also indicating that they have a relationship, as well.
Practically every character in Gravitation. When it's convenient, that is.
Misty Brown from the manga Gunsmith Cats. She's definitely gay (throughout the series she has a crush on Rally), and quite openly so, but it's also very heavily implied that she has feelings for Bean Bandit (she tries to make light of this when confronted, arguing that "When a guy takes a bullet for you, everything changes" and assuring Rally that "You're still my first choice").
Possibly, Ippo Makunouchi. He's got a quite sweet crush on Kumi Mashiba and she more or less reciprocates, but his attitude towards his very handsome rival Ichiro Miyata is so tinted with Ho Yay that his friends don't hesitate to say he's "gay for Miyata". And there's his Ho Yay with his also very good-looking Russian friend Volg, too...
Nishizawa Ayumu, who is refreshingly good natured and frank about being a little aroused by her best friend, who also loves her childhood crush.
Wataru's mother. She was frustated because her childhood love married someone else, and that childhood love was no other than Nagi's mother. Afterwards, Wataru's mother married Wataru's father, who she used to love.
Nene in Hyakko, the Class Representative, is openly bisexual—though considering the very small amount of males the cast runs into, and the extremes to which Nene goes with her lesbian side, she could very easily slip into Schoolgirl Lesbians territory at any moment.
Junjou Romantica: Since Miyagi's first love was a woman and he is currently very much in love with a man, he's probably this trope.
Nina Hopkins of Black Butler. In chapter 37 Nina states that she only likes girls, and boys that are 15 years or younger, prompting Sebastian to comment that he does not need to know what she does in her spare time. She is slightly perverse in this regard, openly groping Mey-Rin's breasts and thighs.
Aoi Reiji from the Boys Love manga Love Mode is breaking up with a female lover in the beginning of the series, is identified as Bisexual by his older brother, then ultimately decides the love of his life is Naoya. He affectionately calls him a "brat".
Madlax arranges a date with a (male) guerilla fighter, flirts with a boy she's hired to protect, sleeps cute and cuddles with a woman she's hired to protect, and ends up with her (female) Rival in the finale.
In The Melancholy of Haruhi Suzumiya, none other than Haruhi herself confesses to Kyon that this trope applies to her when she tells him "...as long as they aren't an ordinary human, it doesn't matter if they're male or female".
Asougi Rin from Mnemosyne, definitely, who beds both multiple beautiful women and handsome studs with regular intervals.
Kou from Monochrome Factor who flirts with both Haruka and Aya. (This is an anime-only addition, however.)
Nezumi from No6. He kisses both Sion and a female prostitute.
This seems to be the default in No6, at least for men.
There's a surprising example in Onegai Twins. The flamboyantly gay Shimazaki is actually bisexual; both he and Tsubaki have been pursuing Maiku to avoid admitting they still have feelings for each other.
Haruhi Fujioka notes offhandedly that her gender identity is pretty weak anyway, so she doesn't mind if people think she's a guy. And guys, girls, looks? What's that compared to who they actually are, anyway? Though, in truth she doesn't show anything more than politeness to girls from this point on. At the end of the first episode/chapter, she jokes that it might be fun to have fangirls, adding that maybe she's "a little bent that way". And she doesn't seem to mind flirting with the girls, although she may or may not actually understand that she's flirting.
Haruhi's father Ryouji "Ranka" Fujioka is a male crossdresser and confirmed to be bi in side-materials, but after the death of Haruhi's mother he said he could never love another woman.
George from Paradise Kiss, mentioned in an aside. If standing on top of your car and announcing it loudly can be called that.
Sailor Moon: Implied in the third manga arc with the feelings Usagi shows toward Haruka/Uranus even after finding out that Haruka is a girl.
She's also quite taken with Rei in the first arc. She makes the same heart-eyes at Rei as she does at Tuxedo Kamen, and she makes a point to call Rei her "beautiful" friend whenever she's going through the recap introductions before each volume.
Ryuuki from Saiunkoku Monogatari flaunts his attraction to men in order to show himself as a king who won't produce a heir anytime soon and he once teasingly flirts with his brother in disguise, though Ryuki does not know that badly squicking him. Also it is fairly clear from the beginning that he's falling for Shuurei hard. He has to outright declare his attraction to women and kiss her to make Shuurei realize it, because she masterfully ignores his non-so-subtle gestures.
Ushio, too, ends up with Senri and is shown dating scores of boys - but the latter turns out to be a ploy for Haine's attention, who she's in love with. She even succeeds in guilting her into a (brief) relationship.
Kyouko Mogami from Skip Beat!. Debatable. Kyouko may or may not like Moko in THAT way. She is certainly a Large Ham for whom a baseless Anguished Declaration of Love is not out of place, but the frequency and severity of some of her Moko-induced spasms still leads some fans to wonder about her.
Desslok from Star Blazers (aka Uchuu Senkan Yamato) has slept with at least two women (Starsha and Mela). And yet he has male bath attendants who rub him down vigorously and then clothe him and he hits on his generals. And he had an entire canon song written about how he and Wildstar are "bound together" and "unable to live with it anymore".
The titular Steel Angel Kurumi. She falls in love with both of her masters Nako (the female) and Nakahito (the male).
From The Demon Ororon there's Othello, brother of the titular character, "the most eccentric playboy" of the royal family of Hell. He was previously married to and madly in love with a woman, Futaba, and currently obsessed with the tragic bounty-hunter Mitsume.
In Toradora! Minori Kushieda suggests to (her crush) Ryuji at one point that she might like girls, but backs off quickly. As usual, it's hard to tell how serious she is, although her feelings for Taiga become painfully obvious after a while.
Especially if you consider that Minori possibly being Bi is the main reason she pushed for Ryuji to go with Taiga over herself, because she loved them both and didn't want either of them to get hurt if they didn't end up together; which they did in the end.
Trinity Blood: Jane Jocelyn, one of the candidates for Albion's throne. She's apparently had seven husbands, and is not even remotely shy about showing her blatant, leering interest in Esther.
Julio from Zero no Tsukaima is in love with the female lead Louise, but he has also expressed an attraction to Louise's boyfriend and Familiar Saito. In the second series, he even tells Saitou before the other takes off to fight a whole army, asking him to take care of an unconscious Louise that he loves him. He also said that he wanted to kiss not only Louise but Saito as well, during a sword fight with Saito.
Sumire from the Venus Versus Virus manga. She has a male love interest, but is quite noticeably in love with Lucia. She eventually says so herself, after a bunch of Ship Tease, though considering the nature of the series it comes off vague
In one Lucky Star strip Konata more-or-less mentioned she doesn't want to be a lesbian.. But she just happens to be one. In another strip though, she had said she wasn't a lesbian. She also shows interest in boys.
School Days: During the Harem Ending of the game, Kotonoha gives Sekai a rather tender kiss after the Rooftop Sex scene between them and Makoto.
Mayo Chiki!: Kanade says that Subaru was her first love and she kisses Jiro.
Midori no Hibi: Kouta Shingyoji. He's a childhood friend of Midori's and also a fellow first year at Ogurabashi High School. He was always in love with Midori, but over the course of the series he manages to develop "feelings" for Seiji that make the miniature Midori extremely nervous.
Prism: Megu's teacher cheerfully explains to her that she swings both ways when Megu goes to her for relationship advice.
Black Canary was going to quip that she was "75% heterosexual" when fighting the scantily clad Talia. Placeholder dialog confusion (not, as many believe, Executive Meddling or Gay Panic) accidentally made the line change to "heterosexual to the bone" in the final printing, much to writer Gail Simone's chagrin.
Also Black Cat from Spider-man has been known to have both boyfriends and girlfriends, and in fact is married to a woman in Spider-Girl.
Moondragon (sort of). She tried mating with the original Quasar, then started seeing the newer more female Quasar, though that could be just her trying fulfill a prophecy. She and and the new Quasar (Phyla-Vell) were lovers before the latter was Quasar, and Moony had had a lesbian tryst (with Marlo Chandler, Rick Jones' wife) before that. Moondragon even said that she didn't feel any true attraction to the men she had tried to attract in the past, so this is a debatable case.
Rictor from X-Factor. He and Shatterstar are currently in a relationship..
Painkiller Jane has in recent comics, slept with a woman on a nude beach, and kissed her best female friend.
In Empowered, Sistah Spooky has had relationships with both sexes. So far, Ninjette's only known confirmed sexual encounter (which occurred prior to the start of the series) was with another woman, though she is obviously attracted to men (*cough*Thug Boy*cough*) as well. Emp is said to be a "three-drink bisexual."
When Ramona's Psycho Ex-Girlfriend attacked him, Scott Pilgrim didn't understand why. It took him a while to figure out that he was dating a bisexual. And he didn't handle it well.
Chester and Deck of Havoc Inc, if Chester's flirting and Deck's cyborg ex-fiance are any indication that they're not just gay.
Catman of Secret Six fame, according to Word of God. There were enough hints that some already suspected it, especially his Ho Yaytastic relationship with Deadshot, which Gail Simone says was as obvious as "a neon sign." His bisexuality supposed to be revealed in the main story until it got Cut Short by the DC relaunch, and Simone says she plans to bring it up the next time she gets to write him.
After years of hinting, League of Extraordinary Gentlemen's Mina Murry's bisexuality was confirmed in Century: 1969, in which she seduces Julia Gallion. Before that she was also in a relationship with Orlando, who can change gender at will.
The second storyline in the Vertigo ComicsMadame Xanadu title featured a flashback in which Xanadu has a female lover in the 15th century.
Possibly Shadow Snark as well given Shine Down's outburst
Shine Down: YOU DON'T UNDERSTAND! YOU'RE BI!
Common in Rainbow Double Dashs Lunaverse. For example, in the story An Early Reunion, Princess Cadenza casually makes mention of having had three husbands and a wife over her then-seven hundred years of life. There are other examples, too, and it is notable that in the Lunaverse, homosexuality and bisexuality are never treated as being any more or less unusual than heterosexuality. The universe's creator has stated that while ponies tend towards heterosexuality, they are usually much more willing to experiment than humans.
A Growing Affection does this with Shizune and Hana Inuzuka. Both have primarily had relationships with men, but they fall in love and are married by the end of the story.
In the Discworld story The Graduation Class, the sexually adventurous Quirmian Assassin, Emmanuelle-Marie Lapoignarde les Deux-Epées, is in trouble and looking for a new career. (this is part of the back-story explaining how she came to be recruited and trained as an Assassin). She is being interviewed by Mrs Rosie Palm for her suitability to follow the vocation of Seamstress. Rosie raises the possibility she could go on, er, a special list for lady customers with sophisticated tastes.
"We would not ask you to do anything you find distasteful or against your nature. What is your attitude on, for instance, Ephebian Island lifestyles?"
Emmmanuelle looked blank for an instant. Rosie tried another euphemism.
"Sapphic practices. L'amour entre des filles."
"I've dabbled. But it is not really to my taste. A small but significant something is…missing."
"A shame. Or you could have gone onto a really special list we direct to female clients of the Guild who are of, shall we say, refined tastes. I can never get enough ladies onto that list, which is a shame, as it commands really premium prices. Supply and demand, you see. Ah well, let me know if you change your mind!"
Dakota Block from Grindhouse is cheating on her husband, but the fact that it's another woman she's having the affair with is overall a fairly unimportant piece of information, as her lover (played by Fergie) is essentially a non-character who gets killed early on. She ends up dating Cherry Darling, the one-legged stripper.
Alluded to by Summer in (500) Days of Summer, who briefly mentions having been in a long-term relationship with a girl in the past.
In Easy A, Olive's mother mentions that she slept with a whole bunch of people when she was young - mostly guys. And her dad casually mentions that he was "gay for a while".
Bi The Way is actually the name of a documentary detailing this phenomenon.
Willy Spino of the Apocalypse film series is hinted as being this in Judgment when he flirts with some men on his way to entering the court to testify against his half-sister Helen Hannah, who became a born-again Christian. In the previous film Revelation, Willy was hitting upon a blind woman who like him ended up taking the Mark of the Beast.
Margot Tenenbaum from The Royal Tenenbaums has a flash back sequence of all her lovers, most are men, but one was another woman.
Samantha James from Just Friends. She's in love with Chris to a Yandere degree, says she "likes girls" and kisses Darla on the lips.
Hanna: Sophie, or at least she'll try it as the cool thing to do.
In Higher Learning, it's never really made clear if Kristen is actually Bi sexual or just curious. Aside from the merest teasing of a sexual relationship with Taryn (although it's implied that they did), some hand holding, and a kiss (which may or may not have really happened), we only actually see her have sex with men. Of course when she was having sex with her boyfriend she was thinking of Taryn.
Estelle in Barbet Schroeder's hippie film More.
Joe Therrian in The Anniversary Party. The party of the title is for the tenth anniversary of his marriage to Sally, and one of the guests is the male lover he used to live with. He also chases another woman or two in the course of the film.
We're the Millers: Edie is fairly happily married to Don, but admits a few times to having gotten excited when she touched Rose's breasts.
Walter Curnow from Arthur C. Clarke's novel 2010: Odyssey Two is described as being a "cheerfully well-adjusted polymorph" by Heywood Floyd, and it is implied that he is interested romantically in Max Brailovsky, one of the Soviet cosmonauts.
More than implied, it's outright referenced by Curnow and Floyd both, when Floyd tells him to back off. Curnow is peeved, but realizes that Floyd is right when he finds out that quiet girl Zenia is already in love with Max.
Captain Bel Thorne from Lois McMaster Bujold's Vorkosigan Saga may be a bioengineered hermaphrodite but h/she pretty much exemplifies Bi The Way in that it (Bel prefers to go by "it" primarily for the discomfort it causes) had lovers of both sexes but was always monongamous and never played anyone of any sex off of someone of another. Gender was never an issue with Bel and it seemed to become attracted to people based primarily on their status as fellow Fish out of Water. Aral Vorkosigan is the same way; bisexual but subconsciously attracted to soldiers. He's had male lovers in the past, and been happily married for the past thirty-odd years. As his wife says, "[He] was bisexual. Now he's monogamous."
A secondary character in Red Seas Under Red Skies casually mentions in conversation that he'll "try anything once". One or two background characters are revealed as gay in a similar way. Locke is mildly surprised at this particular person's admission, but nobody seems to think it's unusual. It's unclear so far whether this is normal in their society in general or just among the thieves and pirates, who are already going against society's norms somewhat.
In Tanya Huff's The Enchantment Emporium many of the characters, including the main one, are from a family of witches, the Gales. Women far outnumber men and with a few exceptions most relationships are within the clan to both keep their magic powers from being diluted and to keep them a secret. This results in a lot of Romantic Two Girl Friendships.
The vampire Henry Fitzroy in the Blood Books(also by Tanya Huff) is openly bisexual (and promiscuous, though this is mainly because he combines feeding with sex). People have a tendency to assume he's straight or gay, depending on which of his love interests they happen to know.
Fitz, of the Eighth Doctor Adventures. He doesn't seem to be out to any of the other characters, but in one scene, the audience is privy to his thoughts about how much he'd like to have sex with Iris Wildthyme (a female Time Lord... or maybe just some woman with a TARDIS), or, actually, the Doctor. Also, his first scene has him in his home era, the 1960s, idly daydreaming about life as aRadio Drama and gay marriage. What with his Brief Accent Imitations of people such as Greta Garbo, one suspects some of the other characters view him as a little camp... Also, the Eighth Doctor, who, along rather the same lines, is stated to be bisexual in a blink-and-you'll-miss-it kind of way — Sam comments on how she's noticed him checking out both men and women. He also mentions having been "more than friends" with Alan Turing (yes, the real person), gets embarrassed when he accidentally says he "loved Shakespeare" rather than "loves Shakespeare", and has an unusually close relationship with another male character, and a Slash Fic about the pairing received a thumbs-up from the author of that book. He also kisses and has implied but very obvious sex with a water nymph, and gets married to a brothel madame... you can see why the fans joke that he Really Gets Around, can't you? He doesn't really... and, like Fitz, he has some mild but evident camp tendencies. And the Doctor has actually kissed Fitz on the lips, on more than one occasion. They're made for each other...
Sam(antha) Jones, with a blink-and-you'll-miss-it reverence to her playing kinky bedroom games with a female character in the novel Seeing I.
Lilith Saintcrow has two — Dante Valentine and Kaia Steelflower. Dante's motivation throughout the first book is to finally track down the demon that murdered the woman she loved. Later she's creeped out when kissed by another woman, but only because that other woman is the test tube baby of her former lover's genes and may have some of Danny's own genetic material. Kaia's ex-lover owns an inn she visits with her new love interest/unwanted baggage, and they kiss to greet each other; her ex-lover then teases her about how cute she finds Kaia's tagalong.
In the fourth book of David Weber's Safehold series, A Mighty Fortress, Merlin Athrawes takes on a second identity to investigate matters in the Temple Lands. Part of this investigation involves making contact with the resident High-Class Call Girl, and taking part in the services she and her girls offer. Later, Merlin idly wonders if this is a case of being interested in the opposite sex or his original (female) identity being interested in other women.
In I, Lucifer angels (including fallen angels) are bisexual, though their natural celestial form isn't physical. They can take on human forms occassionally. Several angels are mentioned as having been close.
Bree Miller in J. Courtney Sullivan's Commencement, although she considers her relationship with Lara to be a case of If It's You, It's Okay.
In the novelisation of Star Trek: The Motion Picture, parts of Kirk's "denial" that he and Spock are lovers can be read as this. In particular, he mentions that he finds his "best" gratification in women, which would indicate he has some basis for comparison...
Christopher "Kit" Marlowe in Elizabeth Bear's Promethean Tales series. Maybe William Shakespeare as well, although this may be a case of "It's okay if it's Kit Marlowe"
Seregil and Alec, the main characters in Lynn Flewelling's Nightrunner series are openly bisexual (even if it takes young Alec a while to get over his Dalnan prudery). This appears to be quite normal in their chosen society as well as in Seregil's homeland, although most of the other named characters are hetero. It's partly dependant on religion - Dalna is a fertility god and so his priests frown on same-sex pairings; the other gods couldn't care less.
In one of the early Sweep books, Morgan has a "Wiccan mind-meld" session with Sky Eventide and discovers she has feelings for her covenmate Raven. When Morgan brings it up to Hunter, Sky's cousin, he just shrugs it off and says "Sky likes who she likes." Apparently, so does Raven, because the two of them are soon dating, though it's a pretty tumultuous relationship.
Prior, one of the main characters in The Regeneration Trilogy, is bisexual. In the first book he meets the woman who will later become his fiancee. Then the second book opens with him picking up a man in a park and going home with him, and you find out that he's done this quite a lot before. Several of the other soldiers are implied to be bisexual as well, and the books talk a lot about what exactly this meant in World War One England.
Siuan and Moiraine (both women) of The Wheel of Time were (hinted at and later confirmed) lovers in their youth; each now has an arguably Sudden Romance with a man.
Lucy, the protagonist from Oldest Living Confederate Widow Tells All.
In Stephen King's The Stand, it's casually mentioned that Dayna Jurgens is bisexual. Stu Redman is flabbergasted by this.
A Song of Ice and Fire: Daenerys, who deeply loved her husband and only considers men as potential partners, has many chapters randomly peppered with Les Yay sex scenes with her handmaids. No one bats an eye at this.
The rumor that Oberyn "The Red Viper" Martell is bisexual has only been mentioned once in the series and it does not affect his character in any way. To him, it seems like he is only interested in sex in general, seeing how often he comes by it.
Oberyn's paramour also seems to be bisexual, as she's aroused by the idea of the Queen in their bed.
It's also mentioned that they've been looking for a blond third partner for awhile.
Rayojini in Burying the Shadow casually asks a woman to sleep with her where she has previously only shown the reader her interest in men. It is implied that this is normal in her culture, because the woman she asks isn't surprised or uninterested herself.
Pretty much everyone in Tales of the Branion Realm, in which if a character is not married, they almost certainly have a same-sex relationship — and can switch off, for example when they're ready to have children, or conversely are past menopause or their spouse has died.
Mad Scientist Frankie Fossoyeur considers "exclusive love" to be "a ludicrous construction of the Judeo-Christian Patriarchy"
Used in multiple layers in Marge Piercy's novel Woman On The Edge of Time. In an early chapter, both readers and the protagonist Connie believe the character Luciente is male. Connie asks if Luciente likes women, and Luciente replies "The most intense mating of my life was a woman named Diana [...] Mostly I've liked males". There is little reaction to this from Connie, but the trope is invoked again when Luciente is revealed to be female, and the fact that her "most intense mating" was with another woman is never questioned.
In Island in the Sea of Time, Swindapa was captured by the Iraiina during a failed attempt at avenging her wounded boyfriend, but after she's freed by Capt. Marian Alston, she becomes smitten with her. Luckily for her, Alston is a closeted lesbian who is attracted to her, so it all works out quite nicely, even if it does take a long time for the both of them to lay all their cards on the table...
Mollie Bolt, protagonist of Rita Mae brown's Rubyfruit Jumgle is a subversion. She has sex with men, occasionally out of curiosity and also as beards since the book takes place from 1956-67, but identifies wholehaertedly as a lesbian.
Played straighter with Nicole "nickles" Hunsenmeier, adoppted daughter of one of the main characters in Six ofOne.
August and Lilac from Of Fear and Faith both fell in love (at first sight) with members of the opposite sex but have an unabashed attraction to one of their same sex teammates (Elin for Lilac and North for August).
The dolls in The Dollmaker were created as companions for a man, though they seem to have more interest in one another.
Ausiel Montoya in Nerve Zero is the ex of the protagonist's best (male) friend, and is later nonchalantly revealed to have a female ex as well.
Eric Caldwell, the Knight Templar in Mr Blank, though it's unclear as to whether he was merely pretending to be bi for the sake of his contacts.
Live Action TV
In one of the first examples on prime time American TV, the character of Tim Bayliss on Homicide: Life on the Street was revealed to be bisexual in the fifth season.
Alice on The L Word, although she laters falls victim to No Bisexuals. The same could be said for Jenny and Tina.
Willow is not bisexual, she identifies as gay. Having had relationships with dudes in the past doesn't negate that.
There are a number of instances where it would be more logical to conclude that she is a bisexual woman who simply calls herself a lesbian. One, for example, being when she does not think to turn the young man with the magic letter jacket into a girl until after Anya reminds her she's supposed to be a lesbian.
Ziva David occasionally implies that she is bisexual, but since she's never actually expressed specific interest in another woman, it's far more likely that her remarks are purely designed to mess with resident Handsome Lech Tony's head.
In the episode "UnSEALed" the team thought a SEAL's wife had been having an affair with another SEAL. Turned out it was actually with his wife. They never explicitly state she's bisexual, though it's more likely than both women being married to The Beard.
Anna in Chuck. In fact, it takes three episodes from some time apart to establish it: When introduced, she's among a group of otherwise-male coworkers. One says that their friend has the opportunity to have sex with a beautiful woman, 'as none of you ever will.' Anna says "speak for yourself." Much later, she's dating male character Morgan. Some time after she begins, Morgan thinks she's cheating with another guy, and a work buddy says "Maybe it's not a guy," and only then does it become clear that The Powers That Be didn't forget about that one-liner from so long ago.
The first non-pilot episode of Being Human mentioned that Mitchell has "fallen off the wagon" onto men as well as women, and had a threesome with Herrick, his sire. It is not 100% certain that this involved sex, but it seems like a safe bet given what we know. Judging by his lines in the pilot when they were Mistaken for Gay, George doesn't seem to know.
Inara in Firefly. "I knew she took female clients but..."
Alistair of Huge mentions that he prefers to take people as people with no attention paid to gender.
Susan Ivanova of Babylon 5 is suddenly revealed as Bisexual (to the extent it was possible to do so on syndicated TV in 1995, anyway) in an episode that deals with her girlfriend Talia being a brainwashed spy. In a previous episode, she had been shown to have had at least one relationship with a man (and to be willing to consider rekindling it). Word of God confirms that Ivanova and Talia were, in fact, lovers, and that Ivanova was definitely bisexual.
Felix Gaeta. Maybe, it's possible the female Cylon he dated was him in denial.
To be clear, since it was never brought out in the main show: the webisode series "Sleeping with the enemy" showed Gaeta as having an affair with Lt. Hoshi, another male bridge officer. He also had an affair with a Sharon (Eight) cylon during the occupation of New Caprica, but the scenes conveying that ended up getting cut. The only part of this backstory that made it into the regular series was the particularly betrayed expression Lt. Hoshi gave Gaeta when the latter tried to stage a mutiny.
Number 3 and Number 6 also seem rather relaxed about it, sharing a bed with Balthar and each other. It's never commented upon any further. Word of God is that all the Human Cylons are bisexual.
The prequel series Capricaestablishes that the people of the Twelve Colonies did not actually have a concept of sexual orientation and that things like same-sex marriages were not considered unusual in any way (group marriages were also legal, but considered eccentric). Thus nobody felt the need to rationalize their sexuality, they just went with whatever worked for them.
Sister Clarice Willow in Caprica is in a group marriage, and is shown sharing a bed with two men and another woman.
Drew sleeps with a woman who casually mentions that he is her first male partner in some time. He is surprised but gets over it quickly.
Drew: "Oh, you're bisexual! That explains why you were so good with my nipples!"
Drew brings a lesbian friend of his to a wedding, and they both mention to each other that they met someone. Then they both point to the same girl.
Drew himself is in a coma in one episode and must choose leave the Lotus-Eater Machine-y dream world to survive. He asks his scheduler (Mimi!) what's on today's agenda, and he's told "at 2:00, you have a threesome, at 3:00, you have a foursome, and at 4:00... [checks schedule] ...sex with George Clooney." When Drew starts to protest, Mimi says "It's okay, there's no one here to judge." To be fair, it is George Clooney.
Howard Moon too, probably. He spends most of the series interested in women and then falls in love with Vince and declares himself a "Massive Gayist". Also, in an earlier episode he described himself as a "liberal man" when rejecting a man who he thought was propositioning him.
Even though he spends most (probably 90% of the entire series) of his time in a relationship with Mr. Lahey, Randy from Trailer Park Boys likes women too. In fact, as Bubbles says shielding his eyes from the website Randy just opened, he "likes all sorts of stuff".
Discussed in one episode, in which Samantha announces, out of the blue, that she is in a relationship with a woman (played by Brazilian actress Sonia Braga) though the girls, who have apparently never heard of bisexuals, take it as more of a sudden sexual preference switch:
Carrie: "How does that work? You go to bed one night, wake up the next morning and poof, you're a lesbian?"
Miranda: "Oh, I forgot to tell you, I'm a fire hydrant!"
Carrie: "Yeah, I'm a shoe! I always wanted to be one and poof, now I am!"
A younger fling of Carrie's called Sean. He and his friends (one of them being Alanis Morisette) make up a nice little bisexual community.
Jane: "What I could really use right now? Big distractions. Like, tequila shots and a dirty weekend with Brad Pitt. And Angelina Jolie."
In an episode of Peep Show Sophie tells Jeremy she's only had sex with four men: "Four?! I've had sex with more men than that." Jeremy and Super Hans also compete over which one of them can get favours from their male teenage bandmate.
Dean Pelton from Community. "I go both ways." indeed.
The Doctor was first portrayed as bisexual in the Eighth Doctor Adventures novels (see Literature), which carried over into the BBC webcast Scream of the Shalka (in which he and the Master are an item). When the new series started, the Ninth Doctor immediately proceeded to flirt with (omnisexual) companion Jack, and the two shared a kiss in the season finale. Although the Tenth Doctor never mentioned attraction to guys, Eleven is rather overtly bisexual (which many other characters also notice), randomly babbles about men fancying him and happily snogs his male companion Rory for no reason.
Jack: (referring to Rose being affectionate with Mickey) Aw, sweet. Look at these two... How come I never get any of that? Ninth Doctor: Buy me a drink first. Jack: Such hard work. Ninth Doctor: (smiling) But worth it!
William Shakespeare was hinted at during the 2007 season, leading the Doctor to quip that "fifty-seven academics just punched the air".
Tenth Doctor: Yeah, well, you can kiss me later. You too, Frank, if you want.
Steven Moffat recently wrote a tweet confirming River Song's bisexuality.
In Torchwood, Everyone Is Bi in the main cast, although each of them in a different way. Jack is properly omnisexual, Gwen is mostly straight but snogs a woman while under alien mind control, Tosh has one relationship with a woman and several with men (although all are equally doomed due to her Cartwright Curse), Owen is up for anything, and Ianto tells his sister "it's not men, just [Jack]". That said, he's openly interested in men in the novels, and it's strongly implied from what his sister says in Children of Earth: Day Five he's a consummate liar.
The show seems to go to great lengths to keep from actually identifying as such, instead substituting euphemisms "bi-lingual" etc. Though she has been in long term relationships with both male and female characters so she's either this or Omnisexual.
In Friends, Phoebe has shown attraction to women a number of times. Just one example is when she was presented with the hypothetical question of who she would rather have a relationship with between Rachel and Monica and immediately blurted out "Rachel!" without taking time to think.
Mind you, it's shown that when Phoebe lived with Monica as roommates, Monica drove her crazy with her OCD, so this might have factored into things. However, the fact that she took so little time proves this isn't a defense.
In the episode with Ross and Monica's hot cousin (Denise Richards) every time she flips her hair the scene goes into slow motion and Barry White's "I'm Gonna Love You Just A Little More Baby" starts playing. Chandler and Ross (yes, Ross) can't help but stare... and neither can Phoebe.
Phoebe: Say something, say anything. Ask her out, she's not your cousin.
On How I Met Your Mother Lily has, on several occasions, suggested that she's sexually attracted to her female friend Robin, saying "a woman's sexuality is a moving target." However, since Lily and Marshall are so completely monogamous for each other, this never really becomes a plot point.
Hellcats flirted with this. Marti mentioned in passing having a teenage crush on Tori Amos and Alice mentions in one episode that she wouldn't kick Milla Jojovich out of bed.
Apparently Gina in The George Lopez Show. In the episode after Randy dumps Benny, she tells Benny that "you don't need a man to have sex" and makes a suggestive face.
Karen Walker from Will and Grace has expressed attraction to women, and Grace in particular, on multiple occasions. Although, considering this is Karen we're talking about, her sexual preferences drift away from "bisexual" into "Anything That Moves" or "Too Drunk to Tell the Difference," frequently.
Nina Van Horne from Just Shoot Me!, is said to be wrong for Denis Finch's father because "all his life he's had one woman, she's had five". And that's just the one's Denis knows about.
Raj is obviously attracted to women and wants to date them, but he has also shown sexual and romantic interests towards men. When confronted about his preference in "The Transporter Malfunction" he denies being anything other than metrosexual.
Possibly Stuart as well. When Raj visited him after-hours at the comic book store, they seemed to both realize the UST building between them, and quickly made excuses to go their separate ways for the night. After Raj left, Stuart thought about it for a second and said "I could do worse". This line may be a reference to Stuat's low self-esteem, but it's hard to watch Raj and Stuart's burgeoning friendship and not see the writers' subtly pushing them toward each other.
Amy is attracted to both Sheldon and Penny, although her crush on Penny is Played for Laughs due to how awkward her flirting tactics are.
DS James Hathaway of Lewis may be this, from what is implied during 'Life Born of Fire.' He's shown to have 3 female love interests, but his implied religious conflicts and past homophobia could suggest he was closeted, perhaps making him Ambiguously Bi.
Hathaway: What, that there's boys and girls and a nice, neat straight line down the middle, and gay's if you like shoes and musicals and straight's if you read Loaded and eat Yorkie bars? (...) Well have you got some finer definition then, sir?
At the end, he's shown with a copy of Loaded and a Yorkie bar, the latter of which he then gives to Lewis by way of apology and closure.
Jane in Happy Endings - there have been references to her going through a "phase" in college, but when one of her female exes shows up it's revealed she's had serious relationships with women, and downplayed them so her husband wouldn't be jealous.
Sam Marquez in Las Vegas. After it seems like she and Mary may have slept together after frequenting a strip club the night before, Mary contends that neither of them is actually gay. When prompted, Sam responds that she always thought it was the person you fall in love with, not the person's sex.
Ragnar from Vikings is pretty excited and eager about inviting Athelstan to have sex with him and his wife, Lagertha, and seems disappointed when Athelstan turns them down.
In a game series with as much Ho Yay as Metal Gear Solid, it's hard to make any clear conclusions about the sexuality of any character. An exception is the marine commander Scott Dolph, who was a lover of Vamp and had at least one grown daughter. Vamp is not Bi the Way, though. Neither is Volgin.
Endrance/Elk of the .Hack series. Both Mia and Haseo become the objects of his affection.
Phantasy Star Portable reveals that Headmaster Nav is an equal-opportunity Dirty Old Man. He just appreciates the "human" form, male or female. He's sympathetic, one of the good guys, and like most Dirty Old Man characters, doesn't go much beyond pinching and flirting.
It is implied that Kanji Tatsumi from Persona 4 potentially is. Completing his social link reveals that he's not, but was afraid that his skill set (including sewing) meant that he was, and he reacted poorly to the implication due to his own uncertainty.
The correct translation was given in the DS release of the game.
They apparently forgot that her healing tech Kiss works on either sex (and apparently robots too)
In the Fate/stay night spinoff-game Fate/EXTRA, both the playable Saber and Caster will fall in love with the protagonistregardless of their gender (though if the protagonist is a girl, Caster will note that she'd be at a bit more ease if the protagonist was a man and that she could always use her magic to "make everything better").
Of the three romances in Jade Empire, two (Silk Fox and Sky) are bisexual.
Leliana from Dragon Age: Origins fits this. She can fall for a male or female protagonist, and has a relationship with a woman in her past (or, at least, a woman she was in love with). She even outs herself quite casually when her disposition is high enough. Likewise, Zevran is also bisexual, as well as a Chivalrous Pervert, though he largely only expresses womanising sentiments unless the player character happens to be male.
One that comes completely out of left field is Branka, Oghren's wife.
In Dragon Age II the creators use the term "subjective sexuality" that basically means that a party member will exhibit interest in the player character's gender if the player character makes a move on them. The only exception is Sebastian Vael who can only be romanced by female characters.
Notably, if you romance Anders as male Hawke, then he mentions that he used to be in a relationship with Karl; if you're female, he never indicates being attracted to men at all. This caused some controversy when combined with his characterization in Dragon Age: Origins – Awakening as a womanizer. Fenris and Merrill are also romanceable by either gender, but showed no real signs of their sexual orientation before the opportunity to flirt with them arises. Isabela, on the other hand, will openly sleep with Anything That Moves.
Dr. Strangelove from Metal Gear Solid: Peace Walker kinda fits into this category. She was mentioned to have a romantic fascination with The Boss, including reviving her as an AI. However, after Zeke was heavily damaged by Big Boss after Paz hijacked it she hints that she has a sexual attraction to Huey Emmerich.
Fallout 2, Fallout 3 and New Vegas all have this option for the player character (though 3 only has it for female characters.) Probably the most prominent NPC who embodies this trope is Cass, who certainly likes guys, but will confess that when she's drunk enough she doesn't care who climbs into bed with her, male or female.
This ended up becoming a functional system in New Vegas; Fallout 3 had the "Black Widow" and "Lady Killer" perks, that gave you a boost to the damage against and additional dialog options with (some NPCs of) the opposite sex. New Vegas added "Confirmed Bachelor" and "Cherchez La Femme" which does it for the same gender, and yes, you can pick both perks applicable.
Garlot (Gulcasa) in Blaze Union, though this is very lightly implied in the other gameshe's in as well. There are two endings to Blaze where he is paired up with someone, and the distinct victors are Nessiah (in the canon ending) and Siskier. In all the other endings, he has a vast Unwanted Harem of both sexes to choose from. The game doesn't go out of its way to address it, but this is still pretty rare for the protagonist of a mostly-linear RPG.
Aphrodite in God of War III will openly flirt with Kratos, but she's not above having a good time with her two beautiful handmaidens.
BlazBlue: Litchi Faye-Ling, who is head-over-heels-in-love with a former research colleague (who is now Arakune), but has been shown to be...highly appreciative of her fellow female cast members...except Rachel and Tao. Mostly when she's drunk.
Though they were all non canon, she has managed to get Noel in very fetishistic Cosplay outfits twice, and Tsubaki once. Then again, that may have been her panda hariclip controlling her...
A not-too unplausible intepretation for Kjelle in Fire Emblem Awakening. She can end up with any of the male kids from the future if you wish so, but in one of the Scrambles she seemed to have a girl-crush on Severa. Then it turned out Kjelle was drunk. And THEN it turned out she had consumed... a Truth Serum instead: she wasn't drunk, she was telling the truth.
Hot Witch Tharja, too. Her attraction to The Avatar is set on stone - regardless of said Avatar's gender.
In World of Warcraft, during a quest called The Day that Deathwing Came: What Really Happened, a male orc named Martek the Exiled must choose a companion from a group of admirers/former lovers - one of whom is a male blood elf. Also, male pandaren Tenwu of the Red Smoke and Ren Firetongue will flirt heavily with you, regardless of your character's gender.
Princess Penny of Dokapon Kingdom doesn't seem to mind which gender the player who wins is. Her father, on the other hand, doesn't want to have two girls married in public.
Persona2 has the protagonist of the first half, Tatsuya Suou. At one point in the game, he is asked who he likes, and gets a choice between any of the party members - however, the main choices are Lisa Silverman or Maya Amano... and Jun Kurosu, a guy whose relationship with Tatsuya is like Heterosexual Life-Partners but decidedly more romantic. All of these options are equally valid and, while Tatsuya has some degree of attraction to Lisa in the backstory and is very upset when Maya dies at the end of the game, two of the game's developers believe that Tatsuya/Jun is the canon pairing. note Jun is not Bi the Way though - he's either just gay or only interested in Tatsuya.
Rin Tohsaka notes that she's attracted to both Shirou and Saber. The Unlimited Blade Works "harem" ending illustrates it; it's not Shirou's harem, but Rin's.
"Huh, I didn't know I swung this way"
Rider has this going on too, having a crush on Ayako in Fate/Hollow Ataraxia as well as being in love with Shirou. In the Hanafuda mini game, Gilgamesh is also revealed to be bi- or pansexual when he meets his younger self.
Caren: I wouldn't have expected any less of you, King of Heroes. As long as it's blonde and has the body of a child, anything's fine right. But, that's a boy you know?
Gilgamesh: Mrph. Fool, don't underestimate me peon! As long as it's cute, it's A-O-K. The true hero is not concerned with the likes of male and female!
The prequel, Fate/Zero, also has a bisexual Rider, which is hardly surpising considering his identity. (Word of God confirms that he had sex with his master, the male Waver Velvet, at very least once). Caster is also confirmed to be bisexual, but his case is a different trope altogether: he mentions that he likes pretty looking boys (like Ryuunosuke) and cute girls (like Joan of Arc.
Dora Bianchi from Questionable Content is by no means in the closet and has very loudly and with a great deal of Squee! checked out girls she likes, but was in a committed, monogamous relationship with a man for months. One of the Fanon's greatest current debates iswas how long it will be before Tai makes a move. (Not that long.)
T-Bob, Kim, Vanessa, and Nancy from Something Positive; even Monette's grown out of her Anything That Moves phase. Vanessa is of particular note. She just mentions how much she loves strip clubs after she started dating Davan with basically no reaction, presumably having mentioned it off scene.
Angela and Heather from Punch an' Pie, the former having also appeared in Queen of Wands. They both deserve special attention for how they managed to do so. Angela came out to Heather by referring to an ex as "he," and Heather's reaction was roughly, "Hey, that's cool, I wish it were easier to do that." Heather herself jokingly strings along a guy who asks if she has a man at home by saying that she has a girl at home. He apologizes for not realizing she was a lesbian, and she corrects him by cheerfully saying that she's "bi...and taken."
Zoe and Nina from Venus Envy. Nina replies to Zoe's admission (forced out of her by Lisa) with "Really? Me too."
Haley Starshine in The Order of the Stick. Her Latent Bisexuality shows up in a discussion between various aspects of her personality, and she once confessed to kissing a girl.
Haley (to herself): "OK, more than once!"
It's also implied she had at least one female lover, mentioning a "Rachel" in the context of people who've left her. Although it's not clear, since that category also includes her mother (who was murdered when Haley was young).
Lisa of Penny and Aggie. Long suspected within the fandom, her bisexuality (after a fashion) comes up during an argument with her best friend Aggie after, due to a misunderstanding, Lisa tries to interest her in casual making out:
Aggie: You could have given me some warning. Told me you were bi. Lisa: I'm not! Usually. I'm just...open. Whatev, you know? Screw labels.
Penny and Aggie themselves go this route eventually as well.
Jamie in Girls with Slingshots. More bi-romantic than bisexual, since she likes girls but doesn't like sex with girls.
Sam, of The Space Between, has apparently had sex with another girl at least once while drunk.
Gin of Gai-Gin regularly ogles the lesbian couple with whom she is friends, but is in a happily monogamous relationship with her boyfriend Pyon (though she's a Yaoi Fangirl to the point that she wished resident Jerk Ass Kitsunefoxy wasn't lying when she tried to spread rumours that Pyon was secretly gay).
Jim and Lorne from Skin Deep, although it's unclear if it's a case of Bi The Way or If It's You, It's Okay, as their other mentioned romantic interests have been female.
Kelli from World of Fizz, she's been dating Dawn since one of the earliest arcs and has erotic dreams about Tim.
Jillian in Erfworld is in love with both Ansom and Wanda, which causes no small amount of difficulty since they're on opposite sides. Wanda isn't above using sex to distract her (male) boss, but Jillian is the only person she's shown anything resembling romantic interest in. Of course, in this world sex has absolutely nothing to do with reproduction, so it might not matter.
Lenny from Our Little Adventure was revealed to be interested in men at some point, but went back to women because he found men too teasing. This might be Played for Laughs as a throwaway joke though...
Anais from Curvy. Is an unashamed Yaoi Fangirl, and has gotten busy with members of both sexes.
Iri, one of the main protagonists of Wayfarers Moon is a perfect example, though her preference is a bit more complicated due to the high fantasy setting: Word of God is that she's only interested in female humans, but elves of either gender seem to appeal equally to her (her only canonically established lovers to date are a human female and a male elf).
Niels has Niels, Peter, and Duncan almost perfectly bisexual (they like guys and girls equally), with Billy leaning strongly towards lesbian but having some interest in guys and the recent confirmation that Agent 300 does in fact like guys, even though he prefers the ladies. Sadly for Duncan, his wife Natalie, although somewhat biromatic, has been confirmed to be firmly in the straight camp.Duncan, although currently out, used to be massively closeted, and is still a little uncomfortable about showing anyone but Niels and Natalie that he likes guys.
Sparrow and Naomi, in Dykes To Watch Out For. Sparrow at one point described herself as a "bisexual lesbian" which raises a few eyebrows. Although given that Stuart is the only male she's been shown to date, it may be more of an If It's You, It's Okay situation.
In When She Was Bad, Tabitha casually reveals to Gail that she's dated both boys and girls in the past.
In The Gamer's Alliance, Ax, Leon and Ronove like both guys and girls. Ax is somewhat confused over this although she has admitted to herself that she used to be quite attracted to Queen Shyla (and still can't quite get the queen out of her mind), Leon ended up disowned when his father found out about him liking both genders, and in Ronove's case the bisexuality foreshadowed that he was in fact a demon because demons are bisexual by nature.
The Whateley Universe has a few of them. Chaka and Fey are those. Sara skirts the line, and is more clearly lesbian. Belphoebe is also this.
Implied by Word of God: Raimi Matthews from Broken Saints. While there is no reference to it in the actual story, writer Brooke Burgess (apart from constantly lampshading the Raimi/Oran Ho Yay) has something interesting to say in the commentary track to Chapter 18, Act 1. When Raimi is getting a tarot card reading from Cielle, The Magician card is involved. Brooke Burgess lists "ambiguous sexuality" as one of the traits Raimi shares with The Magician, saying it's something happening with a lot of young men these days, and he wanted that expressed in Raimi. Of course, that didn't really work, as the only hint that Raimi is anything other than straight is aforementioned Ho Yay.
Nikolai from Sapphire kills two birds with one stone when he tells Alec that both he and Boris were bisexual for one another.
Benzaie, if the jokes about him wanting to touch some guys weren't a give away, is this both in and out of character.
Iron Liz, Nella and Holly (the head of the site) are also bisexual.
As is Jillian of Team Snob.
He may or may not be in reality (plus you have no chance anymore because he's married to his lady), but Doug Walker is very committed to hitting on either gender as much as humanly possible.
On top of him going straight to seductive Puppy-Dog Eyes with Tacoma, cuddling him when he's sick and nearly swooning with glee when he realizes Carl actually cares, Donnie DuPre in Demo Reel had a whole episode dedicated to him having extremely unsubtle affair-subtext-bromances with two men. We even got him asking for Slash Fic and sucking on Pocky as bonuses!
Two thirds of the crew in Guts and Sass: An Anti-Epic are bi, and most of the men come from cultures where no one bats an eye. There is minimal distinction between shades of sexual preference, and no analogue to modern sexual identification.
Diamond, a superheroine from the Global Guardians PBEM Universe was in a long-term committed relationship with another woman (the super-strong heroine Vindicator) for nearly ten years. Within a year after her breakup with Vindicator, Diamond had entered a semi-permanent relationship with the very male Stone.
Regent/Alec from the Web serial Worm mentions having sex with boys and girls in Interlude 10(the nonbonus one).
Family Guy's Lois Griffin makes offhand references to past homosexual relationships that only go on as long as the joke demands and then are promptly forgotten by everyone else as soon as the joke is done. Though she is normally shown lusting openly after men, she quietly works in homosexual behaviour whenever she thinks she can get away with it:
When she didn't believe Meg's conversion to lesbianism, she asked her to prove it by kissing her girlfriend. Unimpressed, she upstaged Meg by kissing her girlfriend far more passionately than Meg had... all in the name of trying to dispense motherly wisdom about being true to yourself, of course.
In "Partial Terms Of Endearment" it's revealed that Lois once experimented with her college roommate Naomi. Which made Peter happy, and hoped for a three-way.
Bill Clinton. The episode where Peter starts hanging out with Bill Clinton. The two hit it off right away, but Peter spends so much time with the guy that he ends up damaging his relationship with Lois. By the end of the episode both of them have been physically seduced by Clinton. The last scene? We get a shot of Peter laying next to Clinton in bed. It should also be noted that Peter had a very shocked look on his face while Clinton had a very satisfied look on his face. Peter then states: "Boy you are good. You are really good."
Hayley Smith has hinted to be this way on occasion, as part and parcel of being a walking stereotype of hippies. Most likely she's still experimenting to annoy her father.
Roger, who comes across mostly as a gay stereotype, subverts this by mostly falling for girls and occasionally showing attraction to certain men. This is usually when Stan is around because the character clash is funnier. The rule seems to be he's only interested in men when it's funny and he's only interested in women when it moves the plot. Or when it's funny.
Roger: Do I like girls? I must like girls.
Francine. Sort of. She has had sex with women before, and seems perfectly willing to make out with a woman to get backstage at a rock concert. But she is primarily straight, compared to Linda Memari.
Harvey from Harvey Birdman, although it is played for laughs, Harvey has slept with males and females alike.
Harley Quinn from Batman: The Animated Series is about as explicitly this as you can get on Saturday morning cartoons. On one hand, her entire character centers around her being in Mad Love with The Joker. On the other hand, she clearly has a thing going on with Poison Ivy whenever she's temporarily on the outs with Mistah J. Word of God confirms that she and Poison Ivy slept together. Since Poison Ivy previously dated men, she's bisexual as well.
Miss Sculptham was intended to end up with someone of the same sex later on in Moral Orel as revealed by a script posted by a creator. Making said character to be one of the few bisexuals in the show to be this trope, as opposed to the Depraved Bisexual status of Clay and Daniel.