4 main title games have had Dark Knights only 2 of those said Dark Knights sacrificed hp to do damage. Xi's with soul eater and Dark Knight Cecil thats it period and guess what Darkness Cecils one skill that they removed for the American release was so weak it wasn't even worth using.
And outside 2 to 3 total fights in the ds remake you used it only once per those fights thats it.
But boy darknight had "always used hp to deal damage" no it hasn't lol.