Did you ever bind your account/was it linked to a google play games account? If not, it is likely gone.
Then you'd have to contact gree with your friendcode and such and see if they can help you in any way. Otherwise, I dont think there's a way you can access it.
The company that owns the software (programmed, I think, but IUGO, but owned - currently - by Gree/Funzio). Look up "Knights and Dragons Gree Support" on Google.