Mittelt is a Fallen Angel who allied herself with Raynare in her plot to steal Asia's Sacred Gear. In the anime, she was ordered by Raynare to stand guard at the back of the abandoned Church. She was soon confronted by Rias and Akeno and fought them alongside Kalawarner and Dohnaseek.

Spoiler

She was terminated by Rias' Power of Destruction after making fun of Issei for how he was treated by Raynare (then as Yuuma) on their first date, which made Rias angry.

(Source: High School DxD Wiki)