amanuensis; ghostwriter; scribe
Refers to a person who writes something for someone else, such as a letter, document, or literary work. Can range from a formal secretary to a ghostwriter.
He worked for many years as a ghostwriter for a famous author.
An amanuensis wrote a letter for the visually impaired person.
Compound of 代 (substitute) + 筆 (writing brush/writing) + 者 (person). The word transparently means 'substitute writing person'.