“I'm running ...”
The answer is on p.6 of the PHB: “The DM narrates the result of the adventurer’s actions.” You’re the DM. So, why are you asking us?
This is not intended to be flippant or disparaging: it’s intended to remind you that you have the DM hat on and one of the jobs that gives you is deciding what happens in response to the player’s actions.
Furthermore, earlier on the same page, it says: “The DM describes the environment.” Klarg is (part of) the environment. Describe him how you like. The fact that he was inspired by the module’s author and the cool picture of him is neither here nor there, he’s your prop now so you get to do what you like with him.
In fact, it’s irrelevant if the author had told you in the text, you get to decide whether to agree with or disagree with the author - you’re the director of this little production which means you don’t have to listen to the script writer if you don’t want to.
It’s also important to remember that NPCs like Klarg are part of the environment and they exist solely as obstacles for the PCs to overcome, or as background colour if they aren’t antagonists. While it can help to ascribe personality and motivations to NPCs it’s important to remember that they are more like props than characters.
Given that the players have “described what they want to do” - interrogate Klarg with the help of. Charm spell, your job is to narrate the result.
First, decide if he knows the answer. If he doesn’t then the players fail. Now, if you take this option then it doesn’t take a genius to guess that the next thing the players will want to do is find out why not.
Second, is this something Klarg would tell them if he wasn’t charmed? Probably not.
Third, is this something Klarg would tell them if he is charmed and now considered them friends. The possible answers are yes (if charmed, he’ll tell them), no (Klarg is the consummate agent and will never reveal a confidence even to his friends), or maybe. If maybe you need to set a DC.
When you have decided this, and depending on the result of Klarg’s saving throw and the player’s Charisma (Persuasion) check, you know how to narrate the results.