Memories - A Mobtale Story - [Mobtale!Sans x OC]

The day a young child was shipped in a crate to the crime ridden streets of the monster sector of Ebbot City, the lives of many people changed.
But not for the first time.
15 years prior, another, slightly older, human child arrived to Ebbot in a similar fashion. The child wasn't quite young looking enough to be a "pup" as one half of the skeleton brothers had become accustomed to calling the new human child; rather, she was older, a bit more understanding of the world around her.
So, unlike Frisk, the older child wasn't found, but instead found others.
Others, being a certain short skeleton, bruised and nearly half dead.
So she helped. The short skeleton, no older than the girl at the time, helped in return.
She met Toriel, the motherly boss, Undyne, the spunky protege, and Alphys, the dear scientist.
The human didn't quite like scientists.
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Years passed, and the group only grew closer, and a familial bond was born. Except for the two that met first.
A blind being could see it. The love they fostered for each other.
But neither acted on their feelings, always aware that they would alway have more time.
Until they didn't.
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The older human disappeared one day.
Only their friends seem know of what happened.
Toriel always seems to have a guilty look on her face.
Undyne is more protective of those she cares for.
Alphys is more withdrawn.
Papyrus is stronger.
...
Sans?
Well.
Sans has never been the same.
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[Present day]
Its been 3 years since then, 2 years since Frisk has come to Ebbot, bringing the group of monsters back together. They all, to some degree, have made peace with what happened:
That is, until Frisk starts saying things too familiar, too similar to be ignored.
Then, the human comes back.
...
"Rowan?"