Yup, you can make up whatever filler you want for Kusagi. As for spirits being related to one another.. I guess its possible. Unlikely, but possible, again its dependent on how you write it. But I will stress that again, the most powerful of spirits I would like most characters to be able to control are high-tier Humanoids. Character's with Gargantuans will be under a considerably larger amount of scrutiny from myself and Rtron