Riou, also known as Leo in manga translations, along with his partner Banikis Gigo, serves as the primary antagonist during the first half of the Faudo Arc. Riou wields Beast Magic, which mirrors some final spells of numerous mamodo that take animal forms.
Besides this, Riou has the unique ability to curse humans, rendering them progressively weaker until they ultimately perish. Only Riou's will can lift these curses. This sinister power has been a part of his family lineage for countless generations, though the exact reasons remain a mystery. However, employing this spell takes a significant toll on Riou’s own physical well-being, especially when used on multiple individuals. To facilitate the breaking of Faudo’s seal, Riou coerces several mamodo bookkeepers into aiding him; those who refuse face death by the curse. Among those affected by this malevolent enchantment were Nicole, Ellie, Alishie, and Li-en.
(Source: Zatch Bell! Wiki)
