This isn't canon, but I just wanted to make an alternative reason why Aichi left.
Certain humans are chosen by the fickle God.
Aichi watched through a glass orb, witnessing Kai starting to smile more and more as he gained more mates, his eyes burning with determination.
They are chosen to fulfill a duty.
He closed his hues, the glass orb dropping out of his hand and shattering, shards of glass scattering and floating away in the rippling blue water.
Mine was to keep the happiness in this world.
Images of smiling friends wafted into his mind, all of them extending a hand towards him, their eyes shining with happiness. Aichi let out a sigh, his eyes drooping as he looked at all of them, examining each and every face.
But I had also made friends.
The crowd of hands grew bigger as more friends appeared, more smiles, more happiness. Aichi gave them all a sorrowful smile.
And then the world was saved, retaining the peace and harmony in this world. My purpose was fulfilled. Yet I couldn't bear disappearing in front of him. So I took the cowardly option to run away.
Aichi closed his eyes, turning around, his back to the friends that were starting to shatter, one by one.
He lifted up his hand, another orb forming as he gazed into it, watching as Kai slipped on his vest and nodded to his mates as they walked towards Fukahara high.
Instead of showing him the truth, I gave him false hope.
"Kai."
Aichi turned around to see the said person who remained, though he was no longer extending his hand. Sorrow expressed on his face as cracks started forming under his clouded green hues.
"Forgive me."
Kai said nothing as the cracks split, the last essence of him shattering and falling into nothingness. Aichi watched blankly, unable to fight back the tears forming in his eyes as he closed his eyes, falling into an eternal slumber.
One day we will fight together again.