A person who gives advice or recommendations, often in an official or formal capacity.
The company brought in a new legal adviser as a counselor.
He advises younger employees as a career counselor.
助言者 is a more general term for someone who gives advice, whereas 勧告者 often implies a more formal or official advisory role.
Compound of 勧告 (advice, recommendation) and 者 (person). The exact historical derivation is straightforward; the spelling is conventionally associated with the meaning.