Yue is the one of two Guardians of the Clow Cards, along with Cerberus. While Cerberus acts as the elector, choosing a candidate to become the next Master of the Clow, Yue is the judge who will test the candidate. Though very little of Yue's critical information is known for certain, it is logical to assume that his blood type is AB, as it is the type listed for his temporary form and is also the type corresponding to his personality according to the Japanese theory of Blood Type Personality (which is used throughout the Cardcaptor Sakura series). Some fans originally believed that because Yue, like many of the powerfully magic characters in the series, doesn't appear to age, that he was immortal, but this theory was shot down by Clow Reed who said in the fourth book of the manga that "every living thing must come to its end", including himself, Yue and Cerberus though they never seem to age or weaken. On the outside he acts very cold and aloof and is much more serious and intimidating than his counterpart, Cerberus.