13(Demon Slayer)

An Au where Fem Zenitsu comes from a very traditional village and reason she was kicked out was because she was molested by an important men in the village and told her parents about it, who didn't believe him as the men always acted kind and sweet in front of others so they kicked her out.

For a few months Zenitsu lived in the streets and hide that she was a girl as she knew that the moment anyone knew she was a girl she would end up violated or sold to the Red District(it could also happen if she had been a boy but people were more interested in little girls then little boys), until she accidentally met Kaigaku who had been traveling back to Jigoro's place after being forced to go grocery shopping by his master.

At first Kaigaku simply planned to shout at Zenitsu for bumping into him but when his lays laid on her, he was reminded of himself before Gyomei took him in and some of the children that had also been in Gyomei's care before Kaigaku fucked up.

He had been trying to make up his mistakes since Jigoro found him and had him open up to him, he even had met Gyomei again and apologized(if Gyomei forgave him or not is up to the writer) so seeing Zenitsu he knew that she was another way for him to make up for what he did and save her from whatever made her live in the streets and look like she did.

Kaigaku took Zenitsu with her, who only accepted because she hadn't been able to get something to eat for a week and even if she ended up dead or abused at least she would probably be able to get some food in her stomach, to see Jigoro who knew immediately that his student had taken the little girl under his care so he simply opened his home and arms for the girl like he had done to Kaigaku.

It took some time for Zenitsu to open up to Kaigaku and Jigoro, including admitting she was a girl and not a boy like they believed plus the reason why she was living in the streets, both master and student were furious with what Zenitsu' parents did but instead of showing their anger, they simply comforted her and gave her their support.

Zenitsu at first did not train to be a demon slayer, they had all been more worried about making sure she was once again healthy but after watching kaigaku and Jigoro's other students train she requested Jigoro to train her which the old man ended up accepting even if he wished for her to choose another future for herself(he tried to offer her to be trained by one of the medics of the Demon Slayer Corps but Zenitsu was stubborn about her decision to fight alongside Kaigaku).

Kaigaku became a demon slayer first then Zenitsu but after a year of him being one, Zenitsu was considered ready by Jigoro and her fellow students and went to the final selection which she passed.

When it comes to her being a demon slayer, Zenitsu decided to hide that she was a girl especially after hearing stories of a some female slayers being given provocative uniforms which she didn't want and she was supported by her master who informed Oyakata about her decision and the why in case someone found out about Zenitsu being a women and decided to start shit.

How this would affect the rest of canon, I'm not sure so I leave it up to the writer to go wild with the changes they want to make including which ships they may add.