Huh... from the Tales games I've looked at, character data was always separate from enemy data. Ditto for non-battle NPCs.
It's probably separate here too. It's not putting Rutee in the party; it's replacing Kratos's model with Rutee's. People have done it before with other characters (you can see Colette -> Swimsuit Sheena in this video too.) You could probably find some way to do something like that in the sprite-based games too (or at least the ones that have the sprites in one piece), though it might take more than AR codes.
And yeah, her model was already in there. It was in the US version artbook.