I just checked the growth charts and it says Nyatchi is wearable and I'm pretty sure that you just hav to get "stuffed animal" and your tamagotchi will be wearing it. The stuffed animal will probably only last until it's time to sleep....
There is no way to get a specific charater, but you do get different charaters based on how you take care of your tamagotchi.
I have seen on many websites that have growth charts, and next to the Nyachi, it says wearable in parenthethis. I would assume that that means you get it by using the costume/stuffed animal. Well, anyways I hope that I helped. ^_^