I don't think there has been anything big about Phione, really, in the anime or games unlike Manaphy. What's special about Phione? And btw, does it have a signature move?
It doesn't have a signature move.
There's really no backstory whatsoever. It's similar to Manaphy is terms of structural appearances, movepool, and abilities. It's basically a weaker form of Manaphy, and that's really all.