A person who helps a new worker learn a new position may be called a trainer, or, with a broader kind of help, a mentor.
The person is a blackmailer.
A knowledgeable person.
a person
A person?
You call that person a scholar.
You would call them someone who peruses.
an ally
You can call someone who does not lie honest, truthful, trustworthy, or sincere.
someone who works for a business
StromClock Ninja
You can call someone who is wise and has good sense as a sage or a wise person.