Kerberotte-chan  

Berotte, Rotte, ケルベロッテちゃん

Kerberotte (also known as Berotte or Berotte-chan) is the main character of the omake "Kerberotte Take Me to Hell!!". She serves as the gatekeeper of hell and her series revolves around her ongoing battle with a fallen angel she's trying to send to the underworld. Despite being physically present in the same world, characters from the primary storyline seem unaware or indifferent towards her. The only individuals she interacts with are Emile, the train conductor, and Flandre (Nakua from the third OVA recognized her image on Sherwood's camera but chose to erase parts containing her for unknown reasons). Additionally, Kerberotte possesses the ability to "jump" between timelines. For instance, she appeared in an alternate future timeline where Hiro sensed her presence but couldn't see her, and similarly, she felt Hiro's presence without being able to visualize him.