I think you're conflating two things: fictional characters/personas, and attributing "music" to them.

People go wild for characters all the time, whether they be Batman, Pikachu, Colombo, Dora the Explorer, whatever -- have you ever seen Nyango Star drumming? https://www.youtube.com/watch?v=-UYgORr5Qhg -- not to mention the entire VTuber scene, where people build the same weird parasocial relationships as other YouTubers do, but hiding behing an animated avatar and voice changer.

Hatsune is an odd one in that she's a character whose "voice" is a set of parameters for a vocal VST (Vocaloid) that you can plug into your DAW and have a synthesized singer along with your synthesized piano, drums, guitar, etc. But for whatever reason, the Japanese loved the character, so they rolled with it and you can now make 3D videos with the character as well as having her singing.

More to the point, the Gorillaz are a full touring band of VTubers, even though the people behind the animated masks are themselves pretty famous already. They have fans, including deranged ones that can't seem to separate the fictional cartoon characters from real people and imagine themselves as Murdoc's wife.

This goes back a long way, I was thinking Josie and the Pussycats, but Wikipedia reminds me that Alvin and the Chipmunks is probably the first.

It also makes me think of ABBA and their "ABBA experience", where they've "digitized" themselves. What it really is is wish-fulfilment and nostalgia; their fans, themselves in their 60s/70s, are thinking of their youth 50 years ago, and the actual members of ABBA also look 70 and not 20 anymore. So they've made a virtual replica of themselves from when they were in their prime, and you can go and dance to them if you want, while the real ABBA members water their garden and feed their cats at home.