It's true that a human life is sacred, but protecting those u care for it makes it all the worth while to protect someone u care about whether they're real of 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 of 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 u 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.