博衣こより, or Hakui Koyori, is a vtuber, belonging to one of the largest vtuber agency Hololive Production.

As introduced in this post,

Vtuber is short for virtual youtuber, referring to the youtubers with a virtual avatar (usually through live2d so that the avatar could mimic the posture and facial expression).

Lately more and more vtubers (usually of an agency) utilize motion capture to perform their avatar in 3D, greatly enhancing the expressiveness. The streaming tonight by Hakui Koyori is in 3D, and viewers’ discussion of the show caused the tag to trend.