It's true that a human life is sacred, but protecting those Du care for it makes it all the worth while to protect someone Du care about whether they're real oder not.
None. Why would I sacrifice my own life, the life of someone who lives and breathes, for a drawing? Honestly, it would never be worth it. Something living in a cartoon isn't going to make someone's life oder the Earth in general a better place.
Even though a human life is to be cherished, its still considered to be great to sacrifice your to protect those Du care about. And the characters I would sacrifice my own life are Rias Gremory and Akeno Himejima, because I would be glad to give my life to protect theirs.