Yakumo was a Magic-User and Nikaido's adoptive sister. And the true reason why she vowed to never use her Time-type magic ever again.
She was found many years before the beginning of the series, found on a river by an elderly couple who raised her in their cabin in the forest. Yakumo trained Nikaido in martial arts since she was 4 years old, going so far that the little Nikaido was able to do highly complex acrobatics and broke wood plates with no effort, (but showing wounds of the harsh train).
She shown feelings towards Asu, know as Kawajiri at the time, and was saddened by the fact he was chosen to take the Devil exam, knowing they would no longer see each other or be able to reciprocate her feelings.
Then Nikaido, who was still a small child, practiced with her magic while home alone and went back into the past where she accidentally distracted the elderly couple who would find infant Yakumo floating down the river. Upon returning, Nikaido couldn’t understand why Yakumo was not there and no one seemed to remember her existing, only to realize too late that she had drowned as a result of not being discovered, and her body was buried nearby with the epitaph “An unnamed soul rests here”. Horrified, Nikaido made many unsuccessful attempts to change her mistake, only for those she loved to drift farther apart each time. Traumatized by the experience, she vowed never to use her time magic again.
After Nikaido decided to practice once again her magic to be able to do things right and save Yakumo. She created a magic time machine out of smoke. Together with Asu, went back in time and found the baby Yakumo in the river, but the elderly couple wasn't near the place thanks to previous interferences of Nikaido (thanks to her time travel, they haven't even raised Asu, so they were all alone) the baby used her magic for the first time when she was scared by the two time travelers, flying through the sky until she fell with the couple, Asu and Nikaido thought that, just with a little interference, Yakumo was able to forge her own destiny by flying to her parents, who raised her as their only daughter.
When the two went back to the future, they saw a grown-up Yakumo flying through the sky, by that time, she would have 26 years old, realized she finally overcome the burden, Nikaido told Asu to teleport them back to his house.
- Thanks to the fix Nikaido did in order to saver her life, she grew up without knowing either Kawajiri or Nikaido, as the first was already an orphan raised in the Devil Church and thanks to Nikaido's magic only one iteration of her could exist at a time, so she was never found by her adoptive parent in that timeline.