Ok, so we know the fire spirits were lying about the virgin blood key. So, what really is the key?
No, that Spirit is not lying. The dark-colored Spirit, speaking in a woman's manner, said to the light-colored one, "I kind of like your nature of cannot tell a lie".
I used to read this manga translations as an easy study of English, but since this ringonda80 dude started translating, It became difficult to study English through this, not only because it is grammatically wrong, but also because all the parts he could not seem to translate began to be replaced by lines of his own creation. For example, the part in this chapter where Lumi says, "I'm a good wife, aren't I?" is altered to "What about the meal!?''. This can no longer be a material for English study, but it is so silly and funny, so I will continue to see how he creates his translations.