-----
I've known for a while now, but ghosts I've risen become something akin to my Spirits. Solid Undead are different, but Phantasmal entities whom I've made a contract or whom I have created myself instantly are registered as "spirits" even though they used to be real people.
I don't really know the details, but they probably have similar skills than I do, such as [Elemental Spirit: Lv--] which makes you literally a Spirit of the Elemental Affinities you possess. This is something I have as well, which makes me a spirit that can use the power of more spirits.
Nevertheless, I merely connected my soul with Silvio and Francesco, and shared their pain and emotions with mine. That way, I was able to channel their Infernal Flames, and use them in conjunction with my own Stats and Mana quantities.