Belphegor's older twin brother and possesses the Storm Mare Ring. He and Bel fought constantly as children, but he always won, so to beat him, Bel put laxatives in Siel's food to incapacitate him. Originally thought to have been killed by Bel, Siel reveals himself as the one responsible for the Millefiore defense in Italy, and can unleash a bat from his special box that can emit invisible storm flames to harm his enemies. His personality is no different from Bel's, except that opposed to a prince, he states that he's a king.
First appeared in Target 220. (A shot of the hand)
Officially debut in the end of Target 221.