The Jade Court are classic Chinese hopping vampires, breath or chi stealers meaning the Jampires have more in common with the Whampires (aside from both being fans of British rock bands).
Reds and Blacks both require blood and the life force suspended in that to feed, rather than take the life force direct.
My theory is Drakul is wearing a Starborn body, Kemmler was after something similar but was too early for Harry’s generation, Justin in this theory was waiting for Kemmler to return, Harry was to be the new body for Kemmler, and with a Starborn body Kemmler had the last piece of the jigsaw for a successful Darkhallow (which is why it nearly killed Cowl), it explains why Kemmler never did the Darkhallow when he could have used the power to crush the White Council, the only Starborn body around from previous generations was already occupied by Drakul, and I doubt even Kemmler fancied taking him on.
It may be that there is a physical limitation on a mortal absorbing too much power unless it is through a construct like a Mantle, or mother Winters Walking Stick, or they are a Starborn.
