Most video games use silent protagonists for the purpose of player immersion. If the player character can't speak, they can't say anything that the player would disagree with. Toby Fox's indie hits Undertale and Deltarune use silent protagonists, but the heroes' lack of lines serve to highlight the difference between the characters and the player themselves. The player characters in Fox's works are shown in-universe to be separate entities from the real-life player, and their silence helps turn that fact into a plot twist – while also making the horrible things they can be forced to do even more disturbing.
Source：https://www.cbr.com/undertale-deltarune-silent-protagonists/
