Daccha! Yes, Lum was originally supposed to be a one-off "monster threat of the week" character, but her overwhelming popularity among fans led to her being rewritten as the main romantic lead.

Dobome-sensei. What do you think about the subject of "childhood sweetheart"? -seonsaengnim in the original KR.

Dropping this on me out of the blue?

Out of the blue, yes.

I'm asking about this in the context of a romcom heroine.

Well... it's a long-running cliché.

I guess you could call it a trope for the losing heroine...

True that. It does convey that impression.

I think they have a term for that too?

They call it the Wester... something.

You mean the Westermarck effect?

That theory where if you're raised together in the same environment since early childhood...

...it becomes hard to develop romantic attraction for the other party.

It's just a hypothesis, albeit a credible one in my view.

I mean, the childhood friend might have a huge headstart in the beginning...

...but if you turn that around, doesn't that mean that's all the progress she got even after spending all that time?

With that glacial pace, it's no wonder why others can catch up and close in from behind.

Like, come on... 40 years ago, wasn't there even a story where the childhood friend lost to a random bit part character who's only supposed to appear for one chapter?

So in the end,

Childhood friend heroines are most fit as NTR fodder in BSS "my unrequited crush got stolen" plots, right? BSS = "Boku ga Saki ni Suki datta no ni", lit. "even though I loved him/her first". Can be considered a subgenre of NTR.