So if I get this right, her spirit can transform into an imaginary twin sister? Because that is fucking awesome! But yeah, you're right there's some things that are gonna need a bit of editing.
The history and concept is totally cool, so no worries there. One issue is the formlessness of the spirit. While it wouldn't matter that much for Bishops and Rooks, I'd really like to have a Knight's spirit to have a shape to work with. The twin sister idea is good and you can keep that as a transformation spell, regardless of what you do with the 'normal' physical apparency of Eight. You could even make it the normal appearance if you wanted to.
Another is the non-existence of firearms (though I might be willing to allows muskets, but I'd have to talk it over with Rilla) I trust that won't be a major issue to edit either way though.
You're also gonna have to pick which stat you're going to use, though. It is either a spell (SPI) or a weapon (POW.) For knights it is possible to use both weapons and spells, but they function using different stats. Goes for both the offensive and defensive spell (although I like the block-idea, seems very versatile)