Hi, I’m looking for stories where a human becomes a character from the show, but is still in our world. Thank you.

Five score divided by Four
The main story, and all the spinoffs are exactly what you are looking for.
PonyEarthVerse, and spinoffs.
This is a huge collaboration, which is probably what you are looking for.
Celestial Disposition - Celestia.
Help I've turned into a princess - Candence
Unlucky Seven - Old Generation MLP Characters I think.
When a Pony Calls - Lyra.
Twilight for a Day - Also has a number of other related stories.
Child of Mine - Cadence.

ponid-21 - Blog - This one definately.
ponid-21 - Story
ponid-21-c Not sure about the other two

There are more, but they don't fit your criteria as well. Like not a specific character or not a character at all, just a pony/ponysona, or only are on earth for a short time, or are not on Earth specifically, but Equestria girls or another crossover earth. or are minor aspects of larger stories.

Child of Two Worlds - Harry Potter.

shinjis-nightmare - Neon Genesis Evangelion, and also short and long dead.

Whip and Wing - Indiana Jones

Altered Visage - Kinda sorta, if you consider generic changeling drone a character, and that it happens later in the story

Also there are a few that are Clop/Fetish stuff
Twisted Transformation Tales - Spellbooks Mind control/TF/Clop, Twilight IIRC.
Becoming Best Princess Twilight Plushie

if you find any more that aren't on this list, let me know, because I find them a lot of fun too.

7552789
PonyEarthVerse is a good group to find these