I don't really know if i have written it correctly but that is what i would like to write right now.
First, what is it? Well, almost every romance has one of them, the childhood friend that (is normally a GIRL) accepts and likes the main character (that usually is not worth of it) even if he is a delinquent, nerd, etc. There are some lines that are common in animes which has a lazzy main protagonist and his childhood friend.
1: "Do you like her?"
2(blush): "No, what are you saying? She... she is just my childhood friend!"
Well, i believe that in the beginnin those lines are quite interesting or even cute, but after listening to that more than 10 times i think anyone would get mad (or not). Osananajimis are a fetish that many people have (like the captain of Onii-chan no Koto Nanka Zenzen Suki Janain Dakara ne!! said in episode 7), but i believe that are quite impossible to exist in real life (i mean, there may be childhood friends, but not as perfect as the one of harens for example).
Just telling, i don't hate them, i even feel pity for some of them. The plot of most animes describe their hardship of showing their feelings for the main character, since their relationship have been always fine only with their strong friendship and that friendship is always what distance them from their loved person... that drama is shown in Otome wa Boku ni Koishiteru where one of the heroines, Mariya MIKADO, tell us how is that feeling.
The best one(also my favorite Yandere), in MY OPINION, is Kaede FUYOU from Shuffle!, she is one of the main heroines and the most convincing of them. During the beginning of the story she is just the devoted childhood friend who lives with Rin TSUCHIMI, the main protagonist of the show. I think evebody that watched it thought he was an fool for not dating such a beautiful and devoted girl, but during her arc it is shown her past and all that happened between she and Rin, and also we discover why both of them do not get closer than that relationship of childhood friends.
I will talk about other childhood friends in another post since i am feeling that this one is getting too long...
Sorry for the problems with the text, i will edit and fix it later...