Sock puppet
Etymology
From the child’s toy puppet made from a sock, worked by a hidden hand. The term migrated to online forums in the early 2000s for a hidden operator "voicing" a false persona.
noun · /SOK PUP-it/
A fake online identity created to praise, defend, or argue on behalf of its real operator while posing as an independent person.
What it names
A specific deception of the internet age — one person wearing many masks to fake a chorus of support.
Example
The author was caught using a sock puppet to post glowing reviews of his own book.
Related idea
Astroturfing is the larger operation — manufacturing fake grassroots support; a sock puppet is one of the individual fake voices used to do it.