Serafall Leviathan, formerly known as Serafall Sitri, is the current Satan with the title of "Leviathan" and Sona Sitri's older sister.

She is a beautiful girl with long, black hair tied in twin ponytails and blue eyes. She also has a child-like body (albeit with big breasts) and usually dresses in magical girl's clothing (from Magical Girl Milky Spiral Seven, the same outfit Mil-tan wears), magic wand and all, prompting Issei to coin her the nickname "Satan Girl" (魔王少女 Maou Shoujo).

(Source: High School DxD Wikia)