Kris didn't know about the spell, Noelle mentions that the voice commanding her didn't sound like Kris. And mentions she can still hear it when Kris goes down in battle.

It's the player's voice.

As a side note, most Kris actions say something like "Kris did X"
But making Noelle do things leads to "You whispered in Noelle's ear"
It's 100% us