honestly when i first saw the dark knight i was thinking something more along the lines of elesis from elsword (as a dark knight/crimson avenger). where she would do attacks that in her attack stance would drain her hp for a huge increase in damage (10% hp for 25% damage boost if i rem right). while also having another stance that would make other abilities give her a percentage of the damage done as HP (25% of the damage returned as health). making it where even though she had almost no defensive skills ... or stats to speak of she was near unkillable anyway...