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Kachans soul laid bare for him to see. Katsumi alive and well, a part of him mourns that the Katsuki he knew and adored will never be around anymore for him to love. Katsumi as sweet and loving as she is, does not have the same cruelty Katsuki exuded as her last life. Missing the spice he had for the former version of his lover had him doing something extra to get that version of him back.
Going through the process of messing with her mind to not miss her human life is one thing. Tricking her to go through the process of transforming into a fae is another. Kintsugi is a more intense process. The small fragments that need to be incorporated into her need more technical help to be completely integrated into his wife's system. Izuku can not just put some drugs into her system and call it a day. Careful process throughout all of Kintsugi to make sure that she is completely unaware to make sure that she is both a fae and still have parts of her personality from her old life.
It is a careful process though. He has to file down some edges of her soul to ensure that they fit inside of her. Nothing to extreme just enough that it fills her up. It can be disorienting but spending time with her physically and exhausting her in bed helps keep her open and plaint for him in the first part. Soon the little fragments of Katsuki can fit inside of her.
What is next is ensuring that they stay inside of her. A type of spiritual glue is then made to fuse the two of them together. It would not be so easy if the two souls did not already go together even if they were processing two different lives. Even so he uses a pieces of ghost tape here and there to integrate the more stubborn pieces. Cleaning takes a minute for him as she gets heat flashes and wants to stay in the cold basement for her to cool down.
In the final step he checks for any gaps in the soul. Her peaceful orange and Katsukis bright red are put together nicely around the gold sticking them together. She is a masterpiece and his careful maneuvering left barely any gaps for him to fill at all. His reward is a more feisty Katsumi, instead of the sweet and more yielding version of her before.
Izuku waits for the pieces to dry before sanding the soul down. Katsumi complained of body aches, so he spent his time either in her soul sanding it down or with her physically giving her a massage and comforting words. When he put on the lacquer to make everything look better and not fall apart, she groans of feeling cold so spent her time outside.
The process finally over, he promises to take her out of the mansion to sight see. As an official fae and have been integrated into her old life soul. He can finally stop the self imposed exile. People may have questions but none that will seriously interrupt their life as the two of them live happily ever after.