Before departing with Riko into the Abyss, Reg warned Kiyui not to look into the mirror on his birthday, fearing he may get Birthday-Death Disease. Kiyui did end up getting sick, although when Mio took him to her ship to treat him, it seemed that he recovered instantly.
Kiyui's exact age is not known, but he has the appearance of a toddler or preschooler. Kiyui has blue, often half-closed eyes and short blond hair with a single curl on his forehead.
He is the only known survivor of the Birthday Death Disease.
Kiyui seems to have formed a special bond with Reg in the time they knew each other before Reg and Riko's decent into the Abyss. In fact, before Reg left, he visited Kiyui in his bed to say goodbye before sunrise.