UnderneathTheWaves said:
http://dic.pixiv.net/a/%E5%8B%87%E8%80%85%E3%81%A8%E9%AD%94%E7%8E%8B

It's still an OC, but 勇者と魔王 is the author's thing that she's from. The character's name is 魔王ベルゼル.

Edit: Sorry, I didn't know someone already answered before me.

Evangeline_A.K._McDowell said:
From an original work by yuusuke. Her name appears to be maou_beluzel. post #826650 isn't the same character, though.

Thank you. Added her to the list.