Kerberotte-chan

Kerberotte (Keruberotte), also known as Berotte or Berotte-chan, is the main protagonist in the omake "Kerberotte Take Me to Hell!!" She is the gatekeeper of hell and her series is about her constant struggle with a fallen angel whom she's trying to send to hell. Although physically involved in the same world, the characters from the main series doesn't seem to know or notice her. The only characters from the main story line she interacted with are Emile, the train conductor, and Flandre (Nakua in the third OVA saw her in the video from Sherwood's camera, but decided to erase the parts of the video containing her for unknown reasons). She also seems to have the ability to "jump" between timelines, such as when she appeared in the alternate future timeline. The future Hiro noticed a presence but can't seem to see her; similarly, she sensed Hiro but couldn't see him.

(Source: Kaibutsu Oujo Wikia)