Leohart/Leohalt is the current Demon Lord who lives in the Demon World, he is also the younger adoptive brother of Liala.

Leohart is a tall young man with tanned skin, purple hair and blue eyes. When he fights his eyes become white and his sclera turn black. He is seen wearing bulky black armor with purple cape. He also has large purple wings. In battle he usually summons his large broadsword.

(Source: Shinmai Maou no Testament Wikia)