The worst of all is the mysterious helper who clearly knows what's going on, but won't say, because the time is not yet right. Sorry. The Time Is Not Yet Right. You have to say it in a portentous, pompous, know-it-all voice, with capital letters.
Imagine you're the hero. You find yourself attacked by weird beasties, and then you're told you have to go to somewhere, retrieve a MacGuffin, and then save the world, but first you will have to face some major opponent, and the fate of the world rests upon you. So of course, you want to know as much as possible before you set off. What the hell are these things? What can they do? How can they be defeated? What's the big bad nasty? Does it have a weak spot? And is there anything else you can tell me to give me an edge? Because, you know, the fate of the world is at stake. It would be kind of useful.
Then this wise guy turns round and basically says, "well, I know, but I'm not telling."
Thanks for nothing, buddy.
And then - even worse - he probably gets killed a few chapters later without divulging the precious secret. It's not like you didn't have enough on your plate what with saving the world, now you have to figure it all out on your own, because this guy who was supposed to be on your side reckoned it would be more cool to be mysterious and withhold vital information from you. That makes him kind of an asshole, don't you think?