A virtual YouTuber is an online entertainer who uses a virtual avatar generated using computer graphics. A growing trend that originated in Japan in the mid-2010s, a majority of VTubers are Japanese-speaking YouTubers or live streamers who use anime-inspired avatar designs. - Wikipedia
A Virtual Youtuber (usually called as VTuber) is a Content Creator, usually on Youtube and Twitch that uses a virtual persona (e.g. a Virtual Avatar) as their main, but not exclusive, presence in their content. For example, Phia, from the Virtual Reality Show, was seen various times with a webcam on both her youtube channel and her twitch, whilst being widely regarded as a VTuber.
A VTuber does not necessarily need to play a character, although that is a common choice. The afformentioned Phia plays herself or, in other words, does not play a character. The same goes for me, JK. Some VTubers play a character with next to no description so that it's ambiguous enough for it to not be