I didn't trust that damned Skull Kid. I knew he was up to no good the moment I saw him!
But, in all seriousness, I didn't really trust Koume and Kotake. I do believe them to be the same creatures from Ocarina of Time, since they share the same names. They could have been putting on some manipulative act to make themselves seem good just so we wouldn't try to kill them. I mean, we don't know what happened to them after Link left Termina. As far as we know, they could have gone on a murderous rampage and killed everybody! How do we know they weren't trying to kill Link by making him take a potion through the Woods of Mystery, in which there are tons of Snappers. Not to mention the fact that they took him to the Deku Palace, even though the King was in an extremely bad mood. Last time I played Majora's Mask, I intentionally drank the Red Potion six times, just to taunt her!
I also didn't trust Romani. I mean, she locked my horse in a cage, and then made me fight with a bunch of potentially dangerous aliens. And all I got in return was a freaking bottle. I mean, I probably could have found one on the Great bay Coast somewhere
EDIT:
I should probably mention that I also don't trust the Happy Mask Salesman, and still don't. Nobody knows what his true motives are, the reason he was in termina in the first place, and also who he is. We know he bears a striking resemblance to the Mask children on the moon, and we also know that he mysteriously disappears at the end. Maybe he knew the entire time that Link could time travel. Maybe he didn't care about Termina at all, and all he really wanted was his precious mask. But why did he want it so bad? Is he evil? Does he in fact have evil intentions? Who knows!! I don't think I'll ever figure that guy out.
Maybe it has something to do with the fact that he looks like Miyamoto. After all, I don't trust Miyamoto much... Aonuma's information is much more credible