Most of the manipulative characters in both fiction and RPGs are absurdly good liars—you know, the type who could sell shoes to snakes or convince a person that the sky was red while standing outdoors at noon on a clear day. But that hides the fact that the people who are terrible at lying can be effective manipulators in their own right—instead of convincing people over to a technical falsehood, though, they tend to focus on getting people to believe things that are true. Those true things just happen to be particularly convenient for the honest manipulator. In many cases, honest manipulators are having to deal with people who have been lied to by someone who is good at direct falsehood, or just have different priorities or different ways of viewing the world than they do.
The first thing an honest manipulator needs is the facts. This isn’t just the things they know to be true about the situations—this is also the things they can safely say they don’t know, or that they know the other person doesn’t know, or that they need to not know.
The next thing she needs is to figure out what it is that her target refuses to believe that she absolutely, positively needs the target to believe. It’s hard to weaponize the truth if you don’t know whether you’re using it as a finesse weapon, a blunt instrument, a hail of arrows or something else entirely. After that (or possibly before that, depending on priorities) is the question of what sorts of things about the situation the target would be invested in finding out to be the truth, or might already erroneously believe, or what conclusions the target is most invested in not reaching; this tells the honest manipulator what her obstacles are, and what points she can tie her own points to in order to make them more palatable.
At that point, it’s mostly a matter of talking to the person she needs to influence—of emphasizing the important or convenient facts, and de-emphasizing the inconvenient ones.
An honest manipulator gains advantage from making her honesty well-known; after all, if she doesn’t or can’t lie in general, why would she suddenly be able to now? In some cases, the key to this is having acquired a reputation beforehand, either for never lying at all, or for never being able to lie without someone catching it. In fantasy universes, an honest manipulator might be able to take advantage of an understanding of the local lie detection magic (most of which, as we know, pings on deliberate falsehoods) to make her honestly clear. Similarly to being bad at lying, there’s gaining a reputation for being terrible at hiding things—if the character has never been able to hide anything before, but manages to keep one tight secret, who’s going to guess it’s there?
The best thing about honest manipulators is that they’re so good at throwing people for a loop. Truth brings people’s guard down and sets up their expectations in a way that lies can’t; use it and the characters who use it carefully, and you’ll have an amazing secret weapon.