27(Demon Slayer)

A soulmate Au where everyone is born with a mark that matches or completes the one in their soulmate's body.

While most soulmates end up being romantic, there are cases of platonic soulmates that end up getting married and having kids due to society's expectation for soulmates to be the best romantic matches as at the time people didn't believe the possibility of soulmates being simply platonic especially those of two people of different gender.

Kanae had never met hers or thought she would met them until she became a demon slayer and found out that another slayer shared the same mark as her after he ended up in the Butterfly Mansion due to an injury he got in his final selection.

While she had not expected to met them, Kanae was excited and wished to talk with them but she was stopped when she was near his room and heard another slayer mention that someone died and later heard the scream and crying from her own soulmate.

It broke her heart hearing her soulmate sound so broken so she choose to wait before trying to approach her soulmate who she later learned was named Tomioka Giyuu.

By the time Giyuu became a hashira, Kanae had gotten Giyuu to trust her but not to open to her which Kanae understood was his way to protect himself from losing someone dear to him so she didn't give up.

Kanae kept approaching Giyuu, talking to him, etc despite everyone saying for her to leave him behind especially when Sanemi joined the hashiras and started crushing on her but Kane did never give up, while she felt attracted to Sanemi the reason why she was trying to hard to be there for Giyuu was because they were soulmates and she wished to at least be his friend.

Giyuu knew that Kanae was trying to befriend him but he feared that if she became close to him that she would die so he tried to push her away but soon gave up as Kanae was very stubborn, of course that Giyuu rejecting Kanae lead to Sanemi disliking him but Giyuu ignored that despite Sanemi's comments sometimes hurting him.

At some point Kagaya noticed the tension between the trio and decided to send the trio to mission together in hopes they would become close or resolve their problems.

In that mission the trio were met with Uppermoon 2 and fought him but Kanae got severely hurt to wish Giyuu ordered Sanemi to take her away and take care of her injuries while he kept Douma occupied.

Sanemi normally would not take orders from Giyuu but as he was worried for Kanae, he did as Giyuu ordered and run far away and started treating Kanae's injuries to the best of his abilities. It was a she did so that he noticed a mark in Kanae's arm that he had seen before when he and Giyuu ended up in the same onsen at the same time, it gave him a new point of view of why Kanae was trying so hard to be there for Giyuu but also made him furious that Giyuu ignored his soulmate so many times.

But he ignored that until he heard Shinobu and kakushis approach, he left Kanae with them and run to go help Giyuu as it was possible to hear the sounds of fighting and there was still time until the sun would come up.

Sanemi was quick to join back into the fight and give a bit of a rest to Giyuu who looked tired from the constant fight and that was not counting his injuries.

At some point as the sun was close to rising and Douma planned to run away, both Sanemi and Giyuu stabbed Douma in a way that he got trapped in the ground due to their nichirins and just before he was able to free himself and run away, the sun was showing its face(so Douma died).

Giyuu collapsed into the ground as Douma evaporated, Sanemi would have done the same but he fought his tiredness to pick up Giyuu and walk towards where he hoped the kakushis still were.

The three hashiras stayed in bed rest for a very long time due to the mission and were put in the same room which lead to latter a confrontation happen(mostly Sanemi demanding to know why Giyuu ignored his soulmate to which Giyuu was confused as he had no idea he had already met his soulmate so Kanae told that she had found out after Giyuu ende din their care after the final selection but she didn't tell him as she knew he was still mourning someone and it ended with the three somehow opening up to each other and growing close).

Kagaya was happy when he noticed that the tension between the trio was gone and that Giyuu had finally his heart to make friends(just a information that Kagaya and Urokodaki gossip about everything but mostly Giyuu).

Giyuu's relationship with the other hashiras and slayers is better then in canon thanks to Kanae and Sanemi(and Shinobu and Kanao are both Kanae's tsugukos so Shinobu did not become a hashira).

Giyuu had been hanging out with Sanemi and Kanae when he was notified of the mission were he would meet the Kamado siblings, and as they had nothing to do the other two went with Giyuu. What did it change? When Giyuu hesitated, Sanemi didn't but he did freeze at seeing Nezuko decided to protect Tanjiro instead of eating him as it made him think of why the same had not happened with his mother.

It took Kanae and Giyuu talking a long time with Sanemi and agreeing that they would let Nezuko live if she resisted his blood but if she didn't, they would kill her(Tanjiro wasn't happy with it but after they explained more about demons to him, he agreed as he knew Nezuko would not be happy to know she killed and eat humans if she ever gained conscience).

Thankfully Nezuko resisted Sanemi blood so Giyuu would guide the siblings to Urokodaki while the other two would talk with Kagaya.

From there things would follow canon in Tanjiro's journey until the trial as now Sanemi and Kanae would be there to defend the Kamado siblings with Giyuu(and while Shinobu went to the mission with Giyuu, it is because while she is a tsuguko she is already hashira level).

What would happen next? It would be up to the writer including if SaneGiyuuKanae end up together(either the three or just two of them) but it would be funny that the trio acted like the Kamado+friends parents so the other slayers started assuming the trio adopted them or something like that(also Sanemi's relationship with Genya would be better then in canon because Kanae wouldn't let Sanemi deny having a younger brother and Giyuu would help him be rational especially when he finds out about Genya eating demons to fight).