It was a casual change.
Perhaps she was able to notice it because of her twin, Hikari.
For example, her daily training had increased in time.
You can see her standing alone in the training ground around midnight. And she started gently opening an old photo album.
From her change in daily routine, such changes in Kaya were slowly but surely causing unexplainable anxiety.

Hikari somehow knew the cause of the change. Kaya’s demeanor changed when she encountered a man named “Hazama” a few weeks ago while visiting the gravesite. Perhaps something happened at that time.
However when Hikari mustered the courage to ask, Kaya answered that there was nothing. The calm days finally returned, but Hikari had a feeling they wouldn’t be around for much longer.
Her premonition was correct, in the worst possible way. Kaya had suddenly disappeared. Except for a single note, Kaya left the Mutsuki family.

A few days after Kaya’s disappearance, Hikari worried about her dearly while trying to act as if everything were normal. Was she to leave the house and search for Kaya, or wait for her return?
Her destination was printed on the note, but not much else provided information of her journey.
Worrying about her sister and her grandparents both, her feet naturally headed to her father’s resting site.
Before, she used to walk with her sister bringing along flowers and a pail of water, but now she was walking alone. She bent her knees in front of the grave and prayed to her dead father on what to do, but he could not answer, and only time passed.

Hikari passes through the gate of the temple as she heads home and sees a man; Hazama.
“How are you, Miss.Hikari?”
Hazama smiles and gently greets Hikari. His figure is very polite and gentlemanly. In a quick instant, Hikari’s face turns fierce and quickly pounces towards Hazama. The paved gravel is roughly trampled upon.
“What did you do with my sister?! Say something!!” Hikaru demands angrily.
“There seems to be some misunderstanding. I merely suggested that she enter the 6th F.F.S tournament that will soon take place. If she left, it’s likely that she’ll join. Did you decide and head for Zone Prime?”
Hikari is taken aback by Hazama, who has a gentle and polite attitude no matter what.

“My dear sister.. Have you really done this upon your own will?” Hikari violently denies the thought, and shakes her head. Hazama, who was analyzing the situation with a thin smile, made a suggestion.

“If you want to pursue your sister, take this. Do not hesitate. If you participate in the F.F.S, you may find Kaya.” Hazama presented a jet-black envelope– an invitation to the 6th F.F.S. Hikari slowly took the invitation from Hazama. Her expression tightens as if she were deciding something. Her hesitation soon left.