Koro is a missable Puppet in the game.I will try to explain how to get it with minor spoilers.

At some point of the story you will have to get a toad for a NPC in Mothergreen, the toad can be found during a fight at Nathalie River (Wonder Woods). After bringing it to him, he will ask you a toad dish which will lead you to find some toad BBQ. Go back to Burg's Restaurant (Wonder Woods).

After the fight you can go back directly in Orange village and check the house with the kid. Talk to every NPCs to know why he left. Go in the village church and check the tomb alone right of the church. Koro will join your party!

Eventually it's just a few steps from unlocking Cat's Eye Mine but it's more accurate.