In 5 Centimeters per Second, Takaki and Akari spent their childhood together, but due to circumstances of life they don't end up together. Made painfully tragic by the fact that everything is realistically played out.
Air Gear: Itsuki, aka Ikki, and Ringo. The two had known each other since they were children, and though they both cared about one another, Ikki only really thought of Ringo as a really good friend while she was so blatantly in love with him it was almost painful. He only reciprocates towards the end, when Ikki chooses Ringo as his Link Turner over Kururu.
Ano Hana The Flower We Saw That Day presents a childhood Love Dodecahedron. Jinta/Jintan and Meiko/Menma like each other despite her being dead for many years, Atsumu/Yukiatsu also has a crush on Meiko/Menma after many years, Naruko/Anaru liked Jintan as a kid and still does as a teenager, but Yukiatsu considers her as a possible Replacement Love Interest and Chiriko/Tsuruko likes Yukiatsu. At the end, Menma finally passes on with the help of her friends, and it's implied that Jintan and Yukiatsu will hook up with Anaru and Tsuruko eventually.
In Arata: The Legend, Kotoha is in love with her childhood friend Arata. However, Arata is oblivious and only sees her as an important part of his "family".
Baccano!: Jacuzzi Splot and Nice Holystone. Their backstory in the later episodes was quite touching: After Nice permanently scarred her face and lost an eye as the result of playing with explosives when she was little, Jacuzzi got a large jagged sword tattoo on his face so she wouldn't be the only one with a distinctive face. They've been together ever since (although Nice still hasn't given up her love of explosives yet).
The two Official Couples in Bakugan, Dan/Runo and Billy/Julie, consist of characters who met each other when they were children. The difference is that Billy and Julie are known to have been friends for quite some time, while Runo met Dan only once when they were playing in the park and didn't realise the boy who was annoying her that day was actually Dan. Near the end of the first series, both pairs become a couple.
Kagerou, the unfortunateYandere. She is said to be in love with Gennosuke since they were kids. Her poisonous powers developed when she was a teenager, though, and that's one of the reasons why she can't marry him.
In the manga, Gennosuke and Oboro were engaged as kids but only saw each other once before being separated for years. In the anime, this is expanded into a very cute flashback scene with them being adorkable kids together. And in the main story, theirs is a Perfectly Arranged Marriage. But they end up as Star-Crossed Lovers.
In Beelzebub, Himekawa has been arranged to marry his former best friend Kugayama since the first grade. The issue is that Kugayama originally posed as a male (since being the heir to the Kugayama family meant she was raised as a guy), and never told Himekawa her true gender nor the engagement, as it was arranged due to Kugayama having a crush on him in the first grade before eventually falling in love with him. Then their friendship fell apart when they were fifteen due to a betrayal on Kugayama's behalf, pretty much making all of it moot until the truth was revealed years later. Himekawa is still confused about the situation and his feelings on the subject are unclear, but he admits that he still cares for her and will always call her a friend no matter what, implying that his feelings for her run deeper than platonic friendship.
Beyblade has Mariah and Ray. They were shown to clearly be close in childhood, and she continues to carry a torch for him after he leaves to explore the world and improve as a blader on his own. At the end of the manga, it's revealed that they got married and had a child.
Yuzu Yamamoto of Bitter Virgin is in love with her childhood friend Daisuke. It is unrequited, though.
Ryuuken Ishida and Kanae Katagiri met as young children, whereupon Kanae immediately devoted herself to him as his Ninja Maid. Her Undying Loyalty remained unshaken even after Ryuuken's mother arranged a Blue Blood engagement between Ryuuken and Masaki. Although Ryuuken genuinely cared for both women, the arrangement was complicated by both Ryuuken and Masaki knowing they were romantically incompatible and by Ryuuken and Kanae knowing they were very romantically compatiblebut not admitting it. Only when the engagement falls through does a relationship between Ryuuken and Kanae finally become viable; they eventually Marry for Love, having a son named Uryuu. Kanae is later murdered by Yhwach, but her death provides the key to destroying Yhwach forever, the cost of which forever destroys Ryuuken's desire to be a Quincy. The epilogue implies that Uryuu, once he learns this truth, also quits being a Quincy to join his father in the medical field.
Renji and Rukia lived in a gang in one of the worst districts of Soul Society. Once the rest of their gang had died, the pair become Shinigami but are separated by Byakuya's adoption of Rukia putting them into very different social classes. In a strong implication of just how much Rukia means to him, Renji spends the next forty years training to become strong enough to defeat Byakuya so that he can reclaim his relationship with Rukia. Renji eventually admits to Ichigo that his loyalty towards Ichigo is because Ichigo is the one who made it possible for him to reconnect with Rukia; he tells Ichigo the only reason why he's admitting it is because the pair are alone and cannot be overheard. The epilogue reveals that Renji and Rukia do get together and have a daughter, Ichika.
Ichigo, Orihime, Tatsuki, Keigo, Mizuiro, Sado and Uryuu are all very close school friends who grow up together to retain that closeness in adulthood. While several of them are also warrior comrades, fighting side by side with the Shinigami, the others know what's going on and are in loop even if they cannot join the fight. The epilogue reveals that Ichigo and Orihime (who met in junior high) end up together and have a son, Kazui, while it's implied that Tatsuki and Keigo either are already together or in their way to be.
Sanae and Tsubasa have known each other since their elementary days, which eventually grown into love when they are teenagers. They are currently married, and she's pregnant with their first child.
Tsubasa's other friend Kumi, is also in love with him, but he rejects her love confession in the sweetest way possible. Kumi is saddened by his rejection, but being a Plucky Girl she bounces back rather quickly.
Yayoi has a crush on Tsubasa, but he didn't return her feelings. She moves on without complications... and might end up with her other childhood friend, Misugi.
A tragic example of this is Stefan Levin and his dead lover Karen, as revealed in the World Youth arc. They have known each other since childhood and they wanted to get married when they grow up and she would've have been his wife if it wasn't for the accident that killed her. In her dying words, she told Stefan to be the best soccer player in the world.
Shuu and Hana in Castle Town Dandelion, where Hana may as well be Shuu's first crush—when he was 10. The proper romance didn't start some 6 years later, as Hana's family moved away, and then back. They're the series Official Couple, and have been engaged.
In Claudine...!...! the titular character's neighbor Rosemarie confessed her love to him but was rejected. She does seem to be the girl who understands him better, out of all of these involved in his life: she calls him "a true man given a female body".
In Corsair, when Aura was young she found Canale lying on the streets and took him in. She's more or less had been planning to marry him ever since, but since this is a Boys' Love series, she ended up on the wrong side of the triangle. She's upset about it, but doesn't push the issue when she finds out.
Team leader Ichigo grew up in the same child unit as Hiro, and she's become heavily infatuated with him, too. Unfortunately for her, she's very much an Unlucky Childhood Friend, as Hiro is Zero Two-sexual. Unfortunately for everyone, her refusal to accept this results in considerable friction within the team. It takes a FranXX synchronization Mind Meld with him to fully realize that he's in love with Zero Two, and that nothing will change that.
Neneko Izumi from DearS is Takeya's childhood friend and near the end she admits she has unrequited love for Takeya, but she had to let him go. She also says she is unable to become Takeya's bride when they are in the classroom talking about their goals in life.
DNA²: Junta's childhood friend Ami Kurimoto wins in the end. (By default, as the one he actually fell for had to return to the future — and she asked Ami to take care of Junta so he'd be happy even if without her.)
Doujin Work: Osana Najimi's childhood friend Justice seems to have feelings for her.
Elfen Lied: Kouta and Lucy. He knew Lucy when they were kids, but after she killed his father and sister because he lied to her about his cousin's gender, he erased her from his memory. However, to Lucy their time together are her only happy memories. As well, Kouta and his cousin Yuka who has a crush on him, and who were friends in childhood. The conflict in childhood between Lucy and Yuka drives a fair portion of the plot.
Erza and Jellal are childhood friends from their days of slavery at the Tower of Heaven. They are the most Ship Teased couple in the series and clearly have feelings for each other. Unfortunately, every time they're about to get a chance to be together something will happen to take Jellal away, like corruption, or death, or arrest, in that order. Even after Jellal escapes from prison, he refuses to start a relationship with Erza since he feels he must continue punishing himself for his crimes.
The anime adds several scenes that attempt to make Natsu and Lisanna out to be this before Lisanna's supposed death. After their reunion at the end of the Edolas arc however, any further romantic development completely stalled, with him focusing more on missions and her focusing more on reconnecting with her brother and sister.
Kenshirou and Yuria. While the story of the Unlucky Childhood Friend Juuza is sympathetic, the story of the victorious Kenshirou is not any better, either. Just as they were about to get married as promised, his jealous best friend Shin suddenly took her away with his iron fist. From then on, Kenshirou was changed forever as a cold-faced stoic. At least, he still managed to reunite with his beloved Yuria in the last moment of her life — miraculously extended thank to his initially Aloof Big Brother, Raoh, who also wanted to grab her into loving him by force...
Another childhood friend of Yuria's, Juuza, fell in love with her very early. Sadly, his hope of marrying her was shattered to pieces when it was revealed that they were half-siblings; to further make it worse, he also discovered that she fell for Kenshirou in the end. Heartbroken, Juuza began to live a life as The Casanova, indifferent to the cause of Nanto Seiken — cheerful, carefree from the outside, but weeping inside — until he learned of the true identity of the Last Nanto General and made his honorable Heroic Sacrifice.
Fruits Basket: Being (usually distantly) related most of the Members of the Zodiac grew up together, so many the Juunishi couples have an element of this:
Haru and Rin knew each other all their lives, but they become especially close after Rin leaves her Abusive Parents when she's ten-ish.
Fullmetal Alchemist: Edward Elric and Winry Rockbell grew up together and is pretty obvious their feelings for each other aren't just familial as they claim. In the last chapter, Ed proposes to Winry in a funny way by stating that in accordance with Equivalent Exchange he will give her half of his life and she will give up half of hers to him and she says she'll give her whole life to him, then rethinks it and offers a percentage back. One of the final images from the series shows them as married with two kids; the Chronicles book released in autumn 2011 further confirms that they go on to have several children.
Hayato and Asuka grew up together since their fathers (who were also best friends since childhood) used to be in their racing team and she even lampshades this once. Their relationship is Like Brother and Sister until towards the end of the TV series, they get engaged in ZERO and eventually marry at the end of SIN.
Canon Immigrants Shiba and Rena are childhood friends and classmates from the PlayStation game. Then in the middle of the game Rena kissed him, and their relationship is established in SIN's ending.
Franz and Eugénie are both in love with their childhood friend Albert in Gankutsuou.
Gate Keepers: Shun and Ruriko. In the sequel Gate Keepers 21, it is revealed that they got married and have a daughter, Ayane. However, Ayane changed her middle name due to resentment with her family.
...and, in fact, the members get to watch Chihiro and Ritsuko enact this trope in Kujibiki Unbalance.
In Gintama, Otose and Jirochou grew up together in Kabukichou. He was a little punk, and she scolded the crap out of him trying to keep him out of trouble. When they grew up, Jirochou became a vigilante to "keep the peace" in Kabukichou, and graciously stepped aside when Otose married Tatsugoro, his best friend and rival (and a cop), because he knew the vigilante life couldn't make her happy. But when Tatsugoro died taking the bullet meant for Jirochou, he makes his friend promise to protect Otose and Kabukichou. It...didn't work out very well.
Kirie from Girls Bravo has known Yukinari since they were children and has a crush on him. However, she is more of an childhood tormenter than friend.
Gokusen: Shinohara-sensei is seen with a cute girl and the Kurodas worry that Kumiko will have a fit. Then they decide that the girl will probably turn out to be his sister (since that always happens in manga). On learning that the girl is actually Shinohara's childhood friend, they go right back to panicking.
Punpun from Goodnight Punpun fell in Love at First Sight with Aiko the moment she transferred. They become something of friends but by middle school have grown apart, however he still pines for her (though he is in love with the idea of her more than her personally). In his twenties he meets Aiko again but their relationship is anything but healthy. They're both deeply troubled individuals and they just keep on hurting each other more and more. The ending of the manga has Aiko killing herself after running away with Punpun when they killed her abusive mother.
This is Gosho Aoyama's thing: Kaitou and Aoko from Magic Kaito, Shinichi and Ran from Detective Conan, Ran's parents Kogoro and Eri, Heiji and Kazuha, Kazunobu Chiba and Naeko Miike and some side characters get in on this. While very few of these pairs are official, it's strongly implied that the others will be, no matter what.
Later in the manga, a schoolgirl named Momiji Ooka appears and claims that not only she knew Heiji when they both were kids, but that he made with her a Childhood Marriage Promise. This potential ramification to the Heiji x Kazuha deal is the center of the plot for the 21th movie too, and there's a scene where Momiji and Heiji are shown as children, making a Pinky Swear... It turned out that Heiji promised *something else* to Momiji, but she decides to not give up on her feelings yet.
Goshuushou-sama Ninomiya-kun: Both Reika and Mayu are childhood friends to Ninomiya and both are still fighting over him years later. Complicated by the fact that he forgot them both because Mayu wiped his memories with a kiss.
Another set: Elle and Beecha. Elle is initially attracted to he3r friend Judau, but she ended up with Beecha instead via a Last Minute Hookup.
Mobile Suit Gundam AGE: Flit Asuno and Emily Amonde. They're Happily Married with two kids in the second part of the series (and one of them is the second-gen protagonist Asemu). However, this mostly happened because of the horribledeath of Flit's first love, Yurin Le Ciel. Flit simply can't forget poor Yurin, and judging by episode 15 Emily is painfully aware of it.
Hand Maid May: Kasumi is a childhood friend whose problem is merely having to compete with a harem full of cyberdolls for Kazuya's attention. It appeared she would win when Cyber-X/Takuya Saotome is revealed to be Kazuya's descendant from the future. The series pulled a twist when it revealed, in the last episode, that Cyberdolls are equiped to conceive and bear children. Talk about "fully functionial"...
High School Of The Dead: Back when she and Takashi were kids, Rei pinky swore that she'd marry him when they were older and used to date him. But they grew apart when Takashi began to ignore her, whereas as his best friend, Hisahi, provided her with the emotional support she needed at that time. Which is how they began dating, all of which was before the outbreak occurred. Takashi was forced to Mercy Kill Hisashi, when he briefly became one of "them". Which left Rei conflicted between her heartbreak over Hisashi's death and her lingering feelings for Takashi. With the world as they knew it having gone to shit, she soon resolved her conflict and decided she'd try to rekindlewhat they once had.
Jeudi and Lundi got together really fast. Their problem with their relationship is less about them getting together and more about them staying together through their ordeals. They do get their happy ending, but not before much heartbreak.
Leon has been in love with Jeudi since they were children. As seen above, he had no chance.
Horizon in the Middle of Nowhere: Toori Aoi and Horizon Ariadust. They were really close friends when they were children, until Horizon died in an unfortunate accident. Years later, Toori meets and falls in love with an automated doll named P-01s who looks just like Horizon. He later finds out P-01s is Horizon reborn in a robotic body. Despite now being an automated doll without emotions and memories, Horizon does reciprocate his feelings.
In the one-shot Girls' Love manga In The Pool, Two OLs Around Thirty one of the women states she's been courting her friend for nearly twenty five years. We're shown her asking her to marry her as a little girl.
Junjou Romantica's Hiroki was in love with his childhood friend Akihiko, who fell in unrequited love with a high school classmate. Hiroki persuaded Akihiko to have sex with him (blindfolded, so that he could pretend he was with the one he loved), which only led to heartache, as it neither won Akihiko's heart for Hiroki nor got Akihiko out of his system. (Both of them ultimately find love with someone who loves them back, fortunately.)
In Maid-Sama!, Shintani Hinata has been in love with Misaki since they were children. Unfortunately for Hinata, he's got no chance against Misaki's feelings for Usui. Later chapters of the manga suggest that Misaki's little sister Suzuna has a crush on Hinata; in the epilogue, they are married.
Tsugumi from Kannagi has a crush on her childhood friend Jin.
Souma Oogami from Kannazuki no Miko is Himeko's childhood friend who is in love with her. Although she does appear to have feelings for him, her feelings for Chikane are stronger.
Kare Kano: Yukino's parents Takashi and Miyako, whom they met when they were children and got married after finishing high school.
Yoshimori from Kekkaishi pines for Tokine — a childhood friend, next door neighbor, and fellow Kekkaishi. Despite their similarities and common histories, Tokine only sees Yoshimori as the crybaby kid she used to watch out for. Tokine also likes older, taller men, and Yoshimori is two years younger than her and rather short at present. Ouch. Later in the manga it is hinted that Tokine seems to be developing feelings for him. Even having a mental He Is All Grown Up moment when she imagines how Yoshimori would be when he's older. The ending of the manga implies they hook up.
Izumi from Kemeko Deluxe! is Sanpeita's childhood friend who harbors a crush on him, but Sanpeita is apparently oblivious of her feelings.
Kemonozume has a love triangle in which Kazuma is an Unlucky Childhood Friend to Rie who is, in turn, an Unlucky Childhood Friend to Kazuma's older brother, Toshihiko. For poor Rie it even involves a Childhood Marriage Promise.
Love Hina: Keitaro made a Childhood Marriage Promise with a mysterious 'promise girl' and the series' overaching plot is about him reuniting with her at Tokyo U. There are two strong candidates for said promise: Naru and Mutsumi, whom he both met as kids around the time the promise was made. Both turn out to be the promise girl, since the three made the promise together. He still chooses Naru, and Mutsumi wishes them happiness.
Played with in the original series. Misa Hayase had a fiancee named Riber Fruhling, whom she had known and loved since she was young (her love for him was one of the two reasons why she joined the military). Some time before the Macross takes off, he leaves and promises to come back, but never does. And in a recognization mission in the Moon base, Misa finds out the truth... Riber was Dead All Along. She's devastated by the discovery, and only Hikaru's intervention keeps her from letting herself die there.
Macross Frontier: Michel and Klan. Michel admits he loves her right before he dies. Apparently they like to make us cry. Interestingly, in the movies, this Tear Jerker is averted in the case of Klan and Michel. And elsewhere this was pulled out of freaking nowhere near the end of the second (and final) movie when Alto flashes back to a chance meeting with Sheryl when they were pre-teens in a revelation that she was the first girl after all.
Macross Plus: Double whammy with Isamu, Myung, and Guld. They have been friends since childhood but their friendship falls apart when it turns into a Love Triangle. Years later when they all meet up again, Isamu finally wins with a flashback that strongly suggests he had already if not for Guld's temper. This nonetheless gives it an entry in victorious (Isamu) and unrequited (Guld).
Mahoraba: Chanohata Tamami is in love with her childhood friend, landlady Aoba Kozue, but doesn't stand a chance against newcomer Shiritori, for the mere fact that her rival is male. Lesbians in anime sure have it tough.
Negi's childhood friend Anya has an obvious crush on him, not that he knows or she wants to admit it since she's a full-blown Tsundere convinced her old friend is being lured into temptation by all those overdeveloped middle school girls.
Ginta and Koyuki in MÄR. Interestingly, while they clearly like each other, them being childhood friends was revealed over time. In the first episode, there's no indication they were anything other than classmates who got along fairly well. However, flashbacks as well as Koyuki's strange dedication to informing his mother of his adventures slowly revealed they've known each other for quite some time (there's flashbacks in the anime dating back to when they were 5 years old). Though Koyuki herself isn't present for much of the series, the Ship Tease lives on with her counterpart, Snow. Even then Koyuki is shown to be very devoted to Ginta, keeping track of her dreams of him and telling his mother of his adventures, and making copies of her school notes for months on end. The anime ends on something of a Maybe Ever After note after Koyuki and Snow merge.
Keiju Tabuki and Momoka Oginome. When he was about to disappear from existence on the Child Broiler, she rescued him from there on her own, at great risk to herself. It is hinted in the series that she had feelings for him as she worked him into her future in the Diary and she would have been his wife if it wasn't for her death. Needless to say, her death had a huge impact on him.
Also, Momoka and Yuri Tokikago. Momoka was the first person who accepted Yuri for who she was and she also took a massive risk to save her, this time via rewriting Yuri's destiny to rescue her from her abusive father, almost burning herself to death. Yuri was also deeply traumatised by Momoka's death, and she's always been in love with her.
Medaka Box: Zenkichi and Medaka. Zenkichi has been trying to get Medaka to fall in love with him for years. Although Medaka's feelings for him were arguable for most of the series, it eventually becomes obvious she does have feelings for him, but thought he "friend-zoned" her after he rejected her marriage proposal when they were two. In the final chapter ending, regardless of the outcome of their duel with each other, they are gonna get married.
In Midori Days, Seiji comes across a childhood friend who had moved away not long after they met. Seiji shows interests in her, but comes to a hard stop when he realizes she's a doujinshi manga artist that writes and drawsBoys' Love works, which is the justification for her not wanting anything to do with straight men.
Naruto's parents, Minato Namikaze and Kushina Uzumaki, met as children, with Minato even admitting the first thing he noticed about her was her beautiful long red hair. They were no more than 14 when they fell in love.
Shikamaru and Temari met when Shikamaru was only 12 and Temari was a few years older. They were enemies at first but became friends rather quickly. It took years for it to florish but they eventually ended up together.
Nisekoi loves this trope, with all the main members of the Unwanted Harem being forgotten childhood friends, one of which the main character was in love with and made a Childhood Marriage Promise to. Turns out the girl who Raku liked in his childhood was Onodera, but in the end he gets over his feelings for her and rejects her because he has fallen in love with his other childhood friend Chitoge.
Safu from No. 6 is in love with her childhood friend Shion. She very straightforwardly confesses her love, he says he thinks of her as a friend but tells her to ask him again in two years, she promises to wait for him... then he runs off with Nezumi, and the series is drowned in Ho Yay.
Vivi from One Piece is implied to be in love with her childhood friend Koza, though it never goes any further than some light Ship Tease moments due to the series having a strict No Hugging, No Kissing policy.
Manami from Oreimo has an obvious crush on her childhood friend Kyosuke, but he doesn't have any strong feelings for her other than close friendship.
Jin and Misaki in The Pet Girl of Sakurasou, but with a twist—while Jin remained loyal to Misaki, he felt unworthy of her, so he became The Casanova. In spite of this, as of the fifth light novel volume, after they graduate, both become a couple.
Serena in the Pokémon XY saga is definitely in love with Ash Ketchum, stemming from a chance meeting when they were younger. Unfortunately, since Ash is a Chaste Hero, she's got a lot of work to do. By the end of the series, and the two part ways (as is tradition for every series), Serena makes it clear that she hasn't given up on him just yet. She tells Ash that he is her ultimate goal, and Serena kissed him as they literally parted. She is the first of the girl companions to give him a kiss (though it's not seen whether it was on the cheek or the lips).
Prétear has fun with this when the resident geek Yayoi digs up the fact that Himeno's stepmother Natsue and her driver Tanaka were classmates. Yayoi immediately assumes there had to be some romance going on between the two since their childhood, with the unlucky one being Tanaka — he got to work for Natsue and stay close to her, but she chose somebody else over him twice. Made even more funny when it is hinted that not only it is not just a product of Yayoi's crazy imagination, but Tanaka actually still likes Natsue.
Psyren: Ageha and Amamiya are childhood friends and they're clearly in love.
Ukyō Kuonji is Ranma's childhood friend and one of his many fiancées. However, Ranma doesn't like her that way; he even thought Ukyō was a boy at first. On top of this, Ukyō tends to act and dress like a guy, so she really isn't helping Ranma think of her as anything other than his boyhood friend.
Mousse has known Shampoo since they were kids, as well, but he's more of a Stalker with a Crush than a friend. Particularly seeing as how it's shown that Shampoo has been ignoring him and beating on him since they were kids together, and has grown even more disdainful of him since. Shampoo does refer to Mousse as "stupid friend from child times" when introducing him to Ranma but this might be a simple slip of the tongue, given Shampoo's notoriouslypoor Japanese skills...
Hanabi and Noriko of Scum's Wish are of the unlucky type, as the person they're in love with has feelings for someone else. And in Hanabi's case, is older than she is.
s-CRY-ed: Ryuho and Mimori meet as teenagers at a high class party their parents attended and quickly bond. The DON'T end up together because Ryuho doesn't feel it would be right, partly due to the last actions of Hopeless Suitor Scheris Adjani.
The Secret Agreement: Yuuichi and Iori meet by chance as kids and despite their different statuses fall in love while they're still quite young, and plan to carry on their affair even after Iori marries. The relationship is undermined a little by the possibility that their love is only a manufactured delusion.
Tanda from Moribito: Guardian of the Spirit has been in love with Balsa since they were children, and they're now in their thirties. She obviously has feelings for him but is apparently too badass to settle down.
Shaman King: Tamao admits that she likes her childhood friend Yoh, even though the chances are slim to none, seeing how Yoh is already engaged to and in love with Anna, who is also his childhood friend.
Paraietta in Simoun is Neviril's childhood friend and has a huge, but unrequited, crush on her.
Sket Dance: Bossun's biological parents grew up together in an orphanage.
Tatsuhiko Aouta from Slam Dunk has had a crush on his childhood friend Haruko since they were young. She likes him as a brother, only, and both has eyes only for Rukawa and has Ship Tease with Sakuragi.
The prequel Piercings reveals that Ryouta and Ayako met when they were both ten years old. Back then, he thought she was a weirdo. In Slam Dunk proper, he's totally head over heels in love with her.
Both Tira and Chocolate Misu of Sorcerer Hunters have crushes (a sweet and quiet one in Tira's case, a much more explicit and open in Chocolate's) on Carrot Glace. They are the only females he doesn't chase after since they were raised together and therefore he views them as if they were his sisters.
In the manga, Carrot marries Tira, leaving only Chocolate to fit this trope. And considering how her sister and Carrot aren't exactly happy after 10 years, it may have been for the best....
Tales of Wedding Rings actually begins with the two childhood friends getting married—in a parallel fantasy world that is the original home of the female friend. Later played with as the marriage was part of a magical ceremony meant to save the world, and there are still four other girls the main character needs to marry.
Telepathy Shoujo Ran: Ran and her childhood friend Rui are in love with each other, although Ran is more vocal about it.
Tenchi Muyo! GXP: Kiriko Masaki has known and looked after Seina since he was a child. She falls in love, and marries him at the end.
Berry and Tasuku from Tokyo Mew Mew's manga sequel A la Mode have known each other since they were children. They at first claim they're Like Brother and Sister, but eventually they realize they are in love. Their love for each other is what breaks the Saint Rose Crusaders' spell of the hypnotizing the city's citizens.
Touch: Tatsuya, Kazuya and Minami were friends ever since they were born and had been together ever since. Almost everybody who knew them wanted Kazuya and Minami to get together and get married, but she doesn't notice his feelings. Then Kazuya dies in an accident and Minami realizes that she's in love with Tatsuya all along.
Tsubasa -RESERVoir CHRoNiCLE-: Syaoran and Sakura grew up together and already have a warm and affectionate friendship in the first chapter. This bond is the most important thing Syaoran has and so this is what he has to pay Yuuko for the power to travel between worlds. Thus, Sakura never remembers any of the time they spent together as children.
In the Mazinger universe, Duke and Rubina from UFO Robo Grendizer were friends and engaged through an Arranged Marriage. They liked each other so neither of them minded. However, her father launched a conquest campaign against Duke's planet, killing thousands of its inhabitants and accidentally turning it into a radioactive wasteland. Then their relationship became a Star-Crossed Lovers romance ended up tragically.
Ataru Moroboshi and Shinobu Miyake. Early chapters of the manga had Shinobu like Ataru but unable to get past his perversion and him being a Weirdness Magnet to become his official girlfriend. Also one chapter early in the series with Ataru's future offspring that was revealed at the end to be hers. They were supposed to end up together, but Rumiko Takahashi's editor found that Lum was unexpectedly popular and Takahashi was forced to change the Official Couple.
Rupa and Carla. Carla has loved Rupa ever since they were children, but he rejected her because he was engaged to Lum and Carla changed from a childhood friend to a violent assailant. At the end, Rupa admits to Carla that he loves her and asks her to marry him.
Ushio and Tora: Ushio and Asako have been friends ever since they can remember and despite their denials, they are very much in love.
Vision of Escaflowne: Merle the Cat Girl has known Van all their lives, she is a friend of his since early childhood, and makes no secret of the fact she's got a crush on him (the main reason she's so hostile towards Hitomi in the beginning, is because she's afraid she'll steal Van from her). Van, meanwhile, seems to do his best to treat her like a little sister and ignore her romantic feelings, falling in love with Hitomi. Merle ends up pulling an I Want My Beloved to Be Happy and chooses to withdraw and leave Van and Hitomi to work things out, though she does give Hitomi a little "talk" about how Hitomi's inability to make up her mind over the Love Triangle.
Nitori and Takatsuki from Wandering Son have been friends since fifth grade. Nitori had a brief crush on Takatsuki in sixth grade but it was unrequited and Nitori eventually began dating Anna. Years later in high school Takatsuki finds herself falling for Nitori after the two becoming distant after middle school. Nitori doesn't return the feelings.
In The World God Only Knows, Keima's friend Tenri has been in love with him since she was 7. However, he refuses to acknowledge her as his childhood friend because she doesn't fit the archetype he's familar with.
Yona of the Dawn: Played with at first with Yona and Suwon but he ends up killing her father and nearly killing her, subverting the trope. It is later played straight with Yona's other childhood friend, Hak.
Your Lie in April: Kousei and Tsubaki grew up next door and they have a close sibling-like relationship, but once Kaori shows up and begins to catch Kousei's attention, Tsubaki realizes she likes him as more than a brother, starting her jealous streak.
Yu-Gi-Oh!: Anzu/Téa and Yugi. Word of God is that, eventually, she would most likely return to Domino from New York at some point and marry him.
Yuya and Yuzu grew up together, and Yuzu clearly has feelings for him. After she goes missing, finding and saving Yuzu becomes Yuya's number one priority. In the end Yuzu's voice alone is able to free Yuya from Zarc's influence and Yuya's dueling gave Yuzu the strength to will herself back into existence once again. The ending heavily implies they end up together as Yuya tenderly welcomes Yuzu home as she cries tears of joy of being reunited with him.
Yugo and Rin were orphans raised together. When Yuzu asks if Rin is his girlfriend, he blushes. The ending heavily implies that they will be together as they are smiling tenderly thought their main counterparts when Rin and the other Bracelet Girls come back.
Yu-Gi-Oh! GX: Ekou and Amon are childhood friends and she is the person he loves the most. However, that love is the reason why Amon has to kill her as a sacrifice to master Exodia the Forbidden One.
Yu Gi Oh Zexal: In the final minute of the final episode, Kotori finally tells Yuma that she "loves his smile", and the usually oblivious Yuma accepts her feelings. It's all very surprising since up until that point, like its predecessors, it's a No Hugging, No Kissing kind of show.
In Yuri!!! on Ice, Yuko and Takeshi have known each other since they were kids learning to skate at the same rink (and they're also both Childhood Friends with Yuri, the main character). Now they're married and have triplet daughters.
YuYu Hakusho: Yuusuke Urameshi and Keiko Yukimura have known each other since childhood, and studied in the same school. Keiko would scold Yuusuke for skipping classes and bad behavior in school, and when they were little kids Yusuke often tried to make her a Childhood Marriage Promise when she got mad at him. They end up together at the end.