Mori Calliope is an English Virtual YouTuber (VTuber) associated with hololive as part of its first-generation English branch of Vtubers alongside Kiara Takanashi, Ina'nis Ninomae, Gura Gawr, and Amelia Watson.

From her hololive profile page:
The Grim Reaper’s first apprentice. Because the world’s medical system advanced so dramatically, she became a VTuber to collect souls. It seems that the lost souls vaporized by the wholesome relationships of VTubers flow through her as well. In the end, she’s a gentle-hearted girl whose sweet voice contradicts the morbid things she tends to say, as well as her hardcore vocals.