It would depend. If the silent protagonist has a defined personality revealed through their canon (be it from conversations with others, implied by other people, actions in canon, etc.), and/or have supplements which help flesh out and develop their personality (For example, if you apped Mario from any of his RPG series where he never actually speaks, or the characters from the Persona series), they are ok to apply!
no subject
It would depend. If the silent protagonist has a defined personality revealed through their canon (be it from conversations with others, implied by other people, actions in canon, etc.), and/or have supplements which help flesh out and develop their personality (For example, if you apped Mario from any of his RPG series where he never actually speaks, or the characters from the Persona series), they are ok to apply!