A significant amount of anime tend to revolve around the relationship of a childhood friend. The common dynamic usually consists of one of them has completely forgotten about this relationship and sooner or later will remember it though some kind of turn of events. The whole childhood friends trope didn't really bother me, but if it pops up more than once in one anime, I will notice it far easily.

The anime that I'm talking about is Kono Naka Ni Hitori, Imouto Ga Iru!, which had introduced two childhood friends nearly 3 episodes in and on the same episode no less. It really makes no sense how someone that interacted with a person for a few days in a hospital would remember them years later. I've watched a lot of anime that don't make sense, but this one probably takes the cake for most shaky character relationship development.

The thing about the concept of the childhood friend is that it is an easy way to have the female character get close to the male protagonist with a reason, but he doesn't really know what it is yet. That disconnect of knowledge between characters and even the viewer also works as comedic, since romantic comedies are pretty popular in anime.

Also, in general, female childhood friends feel the same. First characteristic is motherly, they usually take care of the protagonist maybe out of obligation or simply because they have had a crush on him for years. She has been around the male lead enough to know him well and she wants to be noticed a bit more by him. Which result in her being pretty selfish because of the fact that she has been around longest, to her love interest.

So we get a generic character that I usually don't remember at all, but she more than likely becomes the male's main pick if he doesn't go harem route, or even worse, not choose anyone and leave the series with a non-conclusion. The evolution of romance comedies in anime are probably more to blame for the failing of childhood friends for me, but I guess thats the beast of popularity.

The popularity of childhood friends could also be due to the fetishism of that whole concept, which can be my shortcoming since I don't latch on things that often. Regardless, the more anime that comes out with the childhood friend as a love interest, it becomes more glaring of the dynamic that goes on involving the friend and the male lead.