Envious.
The person is a blackmailer.
A knowledgeable person.
a person
A person?
You call that person a scholar.
an ally
You would call them someone who peruses.
You can call someone who does not lie honest, truthful, trustworthy, or sincere.
someone who works for a business
You can call someone who is wise and has good sense as a sage or a wise person.
You could call them a trigger or a source of envy.