Serafall Leviathan

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).

She constantly embarrasses Sona with her magical girl antics, causing the normally stoic Student Council President to break out in tears of frustration and embarrassment.

(Source: High School DxD Wikia)