I've been looking for a bit now for a rather particular type of story. A boy and girl have been together "forever", and are incredibly close. The boy has been in love with the girl for a very long time, and it's made crystal clear that the girl cares just as much about him. But because they've always been together, she's actually never really stepped back and looked at him as a boy, or considered how much he means to her, or imagined losing him. As a result, he's become a comforting constant in her heart, someone who could make her ach simply by refusing to share his troubles, but also someone who could never make her heart race because he'd never be anything more or less than he's always been She thus looks for, and finds, love elsewhere. The core then is the struggle to resolve the relationship, for the girl to actually see the boy and realize what he means to her.

I've seen references to this sort of story as "cliché", even though I've only seen a handful of series that actually follow this, with a great preference to making this happen only for the girl to realize too late and end up nothing more than an unsuccessful rival or "support" for the final love interest. Please help me find some good stories that actually uphold the supposed cliché.

I can think of two examples, one complete and the other having completed the romantic aspect:
Gacha Gacha:

in the first series, the hero has fallen for his childhood friend, who comes back from a summer vacation changed. He learns her secret: she's somehow absorbed a ton of strange personalities from an experimental VR game; now he has to stick by her and protect her secret, and also protect her from the damage that those personalities may cause to her life. As they're forced to become even closer than they'd already been, her view of him very gradually changes.

Guyver (I've only seen the anime):

Sho has loved his childhood friend's sister, Mizuki, for a very long time, but she has never even thought of him in any such way and instead is crushing on the manipulative student council president. After Sho gains the Guyver and the powers it carries, he desperately struggles to protect her and everyone else close to him, several times coming close to losing his life. Gradually, as events pan out, Mizuki finds her assumptions about Sho challenged while also being forced to face the very real possibility of losing him forever.

hi
take a look in this Category

Vampires are the best!My most favorite is Midnight Secretary but my one and only is Kuran Kaname!

The closest I can think of is the movie of the Tamako Market series, even though there's no real romantic rivalry (as in the heroine had never searched for love to begin with).

In manga form maybe Sakurasaku Shoukougun and Bokukoi Conductor, that share the basic plot. Childhood friends reunite, the guy helps the girl winning over another guy, falls in love with her etc etc. But one of them hasn't been fully scanalated as of yet, and the other one got axed.

Oh, and there was this one shot, called Joyful Days? that might fit, but I don't really remember it that well.

My current top 3:
Fullmetal Alchemist
Pandora Hearts
3-gatsu no Lion

Well childhood love triangles are definitely not the easiest to the frustration and thus the stigma of cliche.

Full Dozer
I just read this one. It's a really good one; had a ball with it (perfect amount of drama, triangle, and youthful true love). Down side would be length because I really wish it was longer (had sooo much fun reading it).

Kakao 79%
Currently ongoing. It's pretty funny but definitely the childhood friends things going on.

There's definitely more so i'll update this post later. The thing about cliche stories is that even though they're cliche, the great authors know how to utilize small plot twists that make for a great story.

Indeed. Tropes become well known to the point of cliché because they can work and work well, it's just that they can also be done poorly. But this "cliché" seems sort of odd simply because wherever I ask for it I generally don't get many answers. I'll get a few, like here, but not that many; certainly not near enough to warrant a term like "cliché". If I asked for damsels in distress or action series with a cool but oblivious guy surrounded by adoring girls who all fight together, the list would overload the server. But when I ask about childhood friend romances, even without any strange requirements besides successful romance, I often don't get all that many, except for cases where the romance is a very minor point. Seems like the real cliché these days is the childhood friend who took too long and missed the chance to win his/her love's heart.

Looking over my collection, I found one other series that looks like it may fit quite well; I've only seen the anime but am looking for the manga:

Nurse Angel Ririka SOS has Ririka and Seiya. The two grew up together and are as close as family. Ririka doesn't really think much of Seiya on the surface, and never seems to consider him in any way except as a friend. But despite this, seeing him in trouble makes her even more upset, in fact much more upset, than her crush Kanou. I don't know how much or how quickly she comes to change, but slipping a glimpse at the Japanese wiki page tells me that I'd like the end.

Ginen Shounen?

Just found a copy of Ginen Shounen and I love it. Please give more.

Chizuru and Ryu from Kimi ni Todoke fit this exactly. They aren't the main couple in the series but they do get loads of development.

I'll try and think of some others~

Watashi-tachi ni wa Kabe ga Aru. seems to (almost) perfectly fit your description.

I second Kakao 79%! The boy has had unrequited love for years, but he can't just confess because she's very vulnerable when it comes to the gossips about them dating (there're reasons for it). And she's dense about whole romance thing, so she refuses to see him as love-interest.

I really liked this webcomic, even though it seemed to me somehow full of clichés in the beginning. Yes, there're lots of them, but I loved main characters in the end. The girl is a genki type, but not annoying and the boy's way too popular among girls and boys, but he doesn't give a damn about it and just wants to be happy with his friend.

There're not many chapters translated into English though. I read all released chapters on comico that are easy to find.

Thx. I'm going slow on Kakao, but it does seem interesting. Takes a little to get used to the art for me, since I'm rather accustomed to the standard, but still nice.