He lives in Gensokyo, working in the Eientei Pharmacy with his mother and the other residents of Eientei. He follows Ayame when she leaves to join the Mistakes crew.
Sometimes there are gaps in Gensokyo's border. Sometimes things from outside worlds get in, sometimes other people. Kaito was one of those things that fell through the border, just outside the House of Eternity. Kaito's the sort of guy that just sort of rolls with weird things that happen to him, so he made himself comfortable. Besides, he was tired of a lot of things back where he came from and was in no rush to return.
Reisen was nice enough to allow him a place to stay, and the two became close. They had a lot in common, despite the obvious differences. Kaito ended up staying a very long time and eventually a child was born to the pair. The young rabbit child was raised in a home separate to Eientei, so that he wouldn't end up as Kaguya's property. Now that he's more grown, he does help out his mother in Eirin's pharmacy.
Ryou's father did end up having to return to his home world, but he does check in frequently.
Comes off as very serious and reserved. He's very polite to everyone he meets, and doesn't really cause much trouble. Doesn't really leave his comfort zone much. However, he's very sweet and affectionate to those he feels close to.
He doesn't really know how to have fun, so he could use some friends to help him loosen up a bit.
Ryou has known Ayame for a good portion of his life. He sees her as a cool older sister, and likes hanging out with her. He trusts her, even though she can be brash and troublemaking.