Zero is my version of Cross.

And while the basic backstory for Zero, is the same as Cross, Zero is seen as a failed version of him.

I believe that there was more timelines than the canon ones that we are shown...

Let me explain, XGaster wants everything to be perfect, right? So, it makes sense that he resets the Monsters, again and again, until he finds the ones that he wants. Zero is techically Cross's prototype.

The canon timelines that we see, happens after XGaster gets the Monsters right.

During his last test run, Zero actually managed to escape Xtale, causing XGaster to recreate Sans, who became Cross.

He didn't mind having to create a new Sans, because Zero had one default, that set them apart from the other failed Sans.

Zero was female.

I know that Sans is canonically male, but to be fair, the Multiverse theory exists, meaning that it's not impossible for a Sans to be female, and we don't have a lot of information on his species, before they were trapped in the Underground, so it is possible that female Skeletons did exist, we just don't see them, or by the time that Undertale happens, they died.

Basically, while Zero shares the same backstory as Cross; having her existance rewritten over and over again, growing up with Xtale Papyrus, working to sell different types of pie, being part of the Royal Guard training, having everyone turn against her, and trying to take the power of Overwrite... However, being a Prototype, and female, causes small ripples of change to develop. These ripples eventually add up, and create a different ending for Zero.

Zero's time comes, just before she takes the human's Soul, at that moment, XGaster Overwrites everything, and he tries to erase Zero.

But being so close to the Human's Soul, when it happens, also leaves Zero Determination Coded, like Geno, and giving her the red scar, and eyelight, that Cross had, after the XEvent.

When it comes to Stats, Zero is not as attack focused as Cross, instead, she's more speed focused, and prefers multiple small attacks, over taking the time to do one big one. She also has a gift for Healing Magic, and basically, is the Cleric of the party, while Cross is the Paladin.

But if Zero almost managed to pull off what Cross eventually did, why didn't XGaster plan for that, and my answer to that is...

He didn't.

Because when he removed Zero, it should have destroyed the path that she was making XTale take, but instead of that, it just planted the seeds for what Cross did.

Because no matter how many times you edit, rewrite, or restart something, there's always going to be a trace of what happened, before the changes were made. XGaster may have uninstalled Zero, and cleared the cache, but no matter how many back-ups you delete, once all of the mods, and added extras are gone, and the system is rebooted, it goes back to it's original settings.

It goes back to zero.

That is why, when Zero leaves Xtale, her name is changed from 'Sans', to 'Zero'.

It's a reminder, that everything must return to where it came from, eventually...

Afterwards, is when things start to favour some of the fanon that I have seen, mainly after Cross gets out, so Zero just lays low, and adjusts to life outside of Xtale.

For Cross, I tend to go towards the fanon route, after he leaves Xtale, aka, Nightmare finds him, and he joins his gang, who are more of a family. I normally try to be canon, where I can, but I like the family aspect more.

Found family Bad Sans is one of my comfort fanfic tropes.

I like the idea of Cross having more family, so most of the time, I have him and Zero being 'twins', with Zero being the main healer/legal grocery runner of the group.

And that's basically it everyone. That's Zero, and how she fits into Cross's story... When Xtale ended, Cross was half an hour older than Zero was, making him older than her, despite the fact that Zero was his prototype. I still haven't figured out how Cross offically meets her, but it's great to have some of my ideas for her, on paper, so that I can focus on my other stories. Maybe I'll mention her, in one of them, someday... But this story/series is where I'll be keeping any oneshots/stories/appearances of Zero.