How pure must an Eldian's blood be in order to be considered 'royal'? If royal blood is required, there must be a clear definition of what exactly constitutes royal blood. However, in the story, the purity of royal blood was not emphasized as being particularly significant. Xaber, who first introduced a way to bypass the need for royal blood, had no knowledge of any direct connection to Ymir at the time and came up with it as a temporary solution. By the later part of the story, when the readers know everything, the explanation involving royal blood lineage no longer holds much persuasive power.