One of Notte’s currently living nine “first-born” children – that is, a vampire he made himself. He was made approximately 150 AD.
Yu is notable in that he’s one of the few first-born Night-Children Notte did not make right away. They were friends for many years first, and Yu, as a result, appears as an elderly Chinese man.
Of course, thanks to the transformation of vampirism, he is anything but frail, no matter how he looks.
Fair warning: dad jokes abound.