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you could shock the world of vampire knight and introduce a new vampire that is resistent to the disease and i has only made her stronger i.e.
(1) she could have new powers i.e. she can be invisble to her pray
(2) she could be love object for kaname or zero
(3) she could be Yuki' sister
I hate to say this, but that's not a good idea at all. You're going to make this OC a Mary-sue, and Mary-sues are a no-no in fanfiction, because they speak of bad writing. If you still want to have this character, at least have her have one of the above traits only, because having all of them would make her too powerful. Have you thought of her negative character traits either? Include that as well, we don't want her to be too perfect.
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If Yuuki is already a vampire, can you make her resistent to the disease? That way she can save the day.
Once again, that would make Yuki a Mary-sue, and though she is not as bad as some of the others I've seen, already in the canon she's starting to show signs of being a mary-sue, though she isn't exactly one yet. If you still want to go ahead with this idea, then you have to give her a weakness as well, like for example if she wants to save people, she can only save the people she cares about the most.