I just bought Sun for the sole purpose of getting Ash Greninja and I just downloaded the demo as well. Is there a way for me to get more than one Ash Greninjas? I wanna give one to my cousin and the other would be mine. Also, how do I transfer it to the main Sun from the demo? And how do I get it in the nature I want?
There are two ways to get multiple Ash Greninja,
Trade with other people
If you have multiple 2DS/3Ds, you can trade the Ash Greninja from the unnecessary save file to your main save file, delete the unnecessary save file and repeat the unnecessary save file to get to a Pokecenter, complete the Sun demo again and transfer the Ash Greninja to the unnecessary file. Rinse and repeat.
Talk to Kukui in the Hau'oli City Pokecenter to transfer Ash Greninja to the main game.
When you are in the main game, you save before talking to the delivery man, and if it's not the right nature, soft reset until it is. (Shift/Select + L + R)
Hope this helped!