Quest Givers in RPGs can fall anywhere on the spectrum from good to evil. But making them believable and worthy of being listened to comes from that NPCs well-developed backstory.
As you are planning out the information this NPC has to give, start by:
Exploring their motivations for wanting to give the information to the players characters.
Uncover how they came about having the information to begin with.
Decide on how the PCs will figure out that this NPC has information they are looking for.
Quest Givers have stereotypically become known for being a bit greedy, if not downright selfish when it comes to giving out information. While this may not always be the case, your players' characters are more than likely going to have to do or give something in exchange for the information.
"But wait," you say. "What if my quest give is an inanimate object like a book, a piece of art, a song, or perhaps even a piece of jewelry?"
In these circumstances, the quest give would actually be the person(s) who were responsible for creating that inanimate object. So, even though your players may never actually meet that NPC, they still had a hand in giving your players a quest.
Besides, you never know what kind of off-the-wall questions your players might ask about where the object came from or why it has such specific information about the player character's pet pig, Henwynn, who just happens to see visions of liches and The Black Cauldron (movie, Disney, 1985 & book, Lloyd Alexander, 1965).