Not so much when the first girl is introduced as how. Does she keep fidgeting with a locket? Is the high school yearbook still on the coffee table after 20 years?

Anyway, my two cents. What a great question.

Only a shown memory from the girl's past.

To me, this comes before the trope

We have a long running animated show, with well known characters with rotating cast that changes every so often. We have the main character (let's call him Aaron), and two rotating characters. In a VERY late season, one of the new cast is Amy. Amy turns out to be Aaron's childhood friend, who has a crush on Aaron.

Aaron doesn't remember Amy whatsoever. Amy has also never been mentioned in passing in previous seasons, nor has ever been referenced to, nor even been implied to be a childhood friend of Aaron. Not even as a possibility.

In dialogue, Aaron could have said, “I’ve had lots of friends when I was younger, but many have moved away.”

This would give Amy the possibility of being Aaron’s childhood friend. We don't get any kind of line like that in the show.

as for the trope, feels out of left field

I kept asking, "Who is this girl? If she's from the MC's past, how come now character in the past seasons including the MC's family, has never mentioned said girl before?"

If,say, long ago Aaron made a little sketch of something, Amy acquired it - out of the trash, some trivial way, kept it and framed it, and still has it. If we contrive a way for Aaron to see it, maybe an aha moment could be written in.

Okay, that's another two cents, I'm up to four cents now.

But, you're right, a Chekhov's Gun would work, or at least help.

Show his increasing disillusionment.
Girl crosses his path (at work related seminar. They are in different fields) She realizes she knows who he is from grade school but it's been years, and they never hung out. But they're contemporaries a couple grades apart and share much of the same experiences, teachers, events...

It turns out she's more genuine than him. There's some attraction, but she's more advanced emotionally. He tries but fails with her, but in the process finds himself again.

Not as something that'd happen VERY late into the series.

Tearing them down with his own slow moral decline based on choices he thinks will improve his life.

You'd have to write the new character so that she becomes the favorite. She's smarter, more mature, more at peace, kinder, and fiercer. When she leaves never to return, the audience wants her back.

But she doesn't. They're stuck with the MC. He has to rebuild himself anew in order to get them to stick around. But it will be hard because the audience agrees with the girl.

Maybe I should've added more detail.

This comes from a TV show. In the 17th season, the new girl becomes part of his new group for said season.

"She's smarter, more mature, more at peace, kinder, and fiercer. When she leaves never to return, the audience wants her back."

Kind of. Within the show, she's kind of new to the scene, trying to figure out what exactly she wants to do. She knows all the basics, of course, but she's unsure which niche she wants to enter.

The MC has changed over the course of the show, getting smarter, wiser, but with each season or two, there's a new challenge he needs to learn.