In the middle of a mysterious forest, two children met while exploring. Though this was the first time they met, they quickly became friends. Eventually they came upon a rocky river bank and decided to rest and get a drink of water. While sitting next to the bank, the oldest child, a boy, picked up a rock and started to throw it towards the water when the other child, a younger girl, cried out to stop him.
The rock in his hand had changed color from light gray to a sky blue with dark gray and flecks of gold. The younger child saw the colors swirling as if they were reacting to something. The boy, however, did not see the colors. The girl took the rock from his hand to see he could see the colors if she held it. The boy saw it change to a deep green with moving yellow specks, like how it looks when you look up into the leaves of a tree. The girl however, only saw a gray rock in her hands.
They picked up multiple stones on the river bank but only that one changed colors. They found the only way for them to both see any color other than gray was when they both held the stone and turned it towards the sky. This changed the color to a golden yellow like the sun. After they both touched the stone, they could both see the color change in their own hands.
The girl took the stone first and tried showing her family. Nobody could see colors when she showed them, and when she could convince others to hold the stone with her, it did not change. The boy tried showing his family as well, but just like the girl's family they too only saw a gray rock.
Eventually the boy moved away and they lost contact until they were adults. The girl kept the rock as a treasure - it was her connection to that boy who she wanted to spend forever with.
When they did meet again, they did not recognize each other. Like when they were children they quickly became friends again. One day, she showed him the stone and he could see the deep green that he remembered from his childhood. He asked to see the stone and once he held it she knew he was the same boy from her childhood.
They eventually got married, and passed the stone down to their children, who passed it down to theirs. For hundreds of years, their family used what they called the soulstone to help them find their soulmate. It eventually was passed to Kuon.
Kuon had the stone with him when he met Kyoko, and he showed it to her to cheer her up. Because she believed he was a fairy, he told her it was a stone for fairies to be able to recognize each other - even if they looked like a human. He knew as soon as Kyoko's eyes lit up when he showed her the stone to cheer her up that she would be somebody very special to him.
He had them both hold the stone together so she would not see the gray when he was around. But eventually, he had to go home. He left her with the stone. Kyoko believed he was from a fairy kingdom. He could not bring himself to tell her the truth - that he was a human just like her. So he left her with the stone hoping that someday they would meet again, like his ancestors. He would be able to tell her the truth then.
Of course, just like Kuon's ancestors, they did not recognize each other at their next meeting. Kuon had become Ren, refusing to be himself around others. Kyoko had locked her heart from love and no longer appeared to be the innocent girl he met as a child. He would recognize their connection first when picking up the stone at the bottom of a stairwell, but it would no longer change colors for him. As soon as Kyoko took the stone back from his hands, he could see the deep green color that he saw last as a child in her hands.
What was going on? Kyoko had acted like it had stayed gray when he held it, so he would act like it did not change colors for her. Maybe it was that he had locked his identity away, or maybe it was because she had locked her heart from love. Whatever the reason she did not see a color when he held it, nothing had to change. He could not afford to become Kuon again. But just the knowledge that Kyoko was the girl from his past still led to change happening. He could not stop being curious about the person Kyoko had become.
The next time he saw the stone, she offered it to him, but Kyoko did not show any signs of seeing a color change when he touched the stone. Kuon thought he saw some gold when she first handed it to him and they were both touching it. It most likely was starting to react because he had decided to let some Kuon out to act as Katsuki. Did she only see gray when he held it, or was she scared to admit she saw a color?
The next time he saw the stone, he did not touch it, but Kyoko did see a color change. When her stalker beagle held the stone, both Kyoko and Ren could see the stone turn a dark black. How could this person affect the stone like that? After Kyoko held it again, Ren could see it was back to the green. Both Ren and Kyoko were relieved to see The black color dispelled from the stone. He still did not let Kyoko know he could see other colors but gray.
Over time, she brought more of his old self out. It was inevitable that she would someday soon learn the truth. When she recognized him as Corn in Guam, he thought that she would finally know the truth. Somehow he was able to keep his secret: that Ren and Corn were the same person. He was not sure how much longer he could keep that secret. It was lucky she didn't bring the stone out that time - he did not want to test to see if it would stay gray like the last time he held it.
The next time he saw the stone, he decided to no longer attempt to hide his true self from Kyoko. When he held the stone after Kyoko handed it to him, the change in her expression was immediate - she finally saw something in the stone when he held it. But her reaction was not what he expected.
"You have to have fairy blood like Corn! It's a darker blue than it was for Corn, but it probably shows a different color for each person."
Of course she would think that. As a child he had told her that the stone was a way for fairies to be able to recognize each other, but you could not see the color change for yourself. He did not tell her what his ancestors' interpretation had been - that they are soulmates.
Maybe he would be able to keep his secret a little longer. At least until they were both ready: for Ren to be ready to be Kuon in front of her, and for Kyoko to be ready to see that her fairy prince had been with her this whole time. In the meantime, he would keep the stone safe, waiting for the day it would lighten to the color she first saw in Kyoto.
