A secretary or personal assistant.
Quora
personal assistant or secretary.
In Kurdish it is called Nama Nus.
A secretary writes letters for a manager.
A person who constantly writes letters of complaint can be referred to as a chronic complainer or a serial complainer.
guider
A secretary
Testator.
The person who writes a check is called the payor. Or in the case of my brother, a sucker, when he writes another check for his girlfriend.
An author.
a publisist a bigraphy
composer
a playwright