1) Marine - Unless she has a split personality, I think it's safe to exclude her.
2) Benoit - He has significantly helped Marine time and again (in more than one occasion Marine would have failed in her quest if not for him) and deeply cares for her. Unless it turns out that he doesn't view Marine in a platonical way then he should be at her side come hell or high water. If he does have romantic feelings, then there is a high danger for him turning to the other side (since Marine DOES see him platonically), especially since there appears to be a chemistry between Marine and Bokden whom he mistrusts.
3) Karine - She spent her days in her home town protecting it from Rogue till Marine comes, so she can't have been a spy or have ties to Mystrock. She believes in Marine as is evident by her fight with Jezabel, so it's highly unlikely that she will betray her.
4) Hirado - As Karine he has spent years in one place protecting it from an evil (Djinn), so he can't have ties to Mystrock. He also seems indifferent to money so I can't see him betraying Marine.
5) Abu - I think it's easy to say that if someone is impossible to turn it's Abu. His nobility as the swamp guardian is established, and he is close friends with Marine's mother to whom she has made a promise to protect Marine. I'd sooner believe Marine having a split personality than him betraying her.
6) Jezabel - It has been shown that she doesn't believe in Marine's cause, but I think it's too heavily foreshadowed to be true. She also doesn't try to play nice as much as a spy probably would. She also isn't thought by Borgon to be worth spending any time or money on as she's described as totally useless. Would be interesting to watch her being tempted by Mystrock if she manages to have a good enough performance at the showdown.
7) Jack - It is theoretically possible to be seduced by May or some other Mystrock cutie, but I think that's how he risks losing a match, not betrays Marine, especially since he is indebted to her for saving him from the werewolf curse.
8 ) James - Same as above. Its worth noting that they did have a motive for spying on Marine (to get enough money to buy the curse stopping potion from the witch), but neither of them seems like the type to offer to kill a teenage girl, as done in the last cutscene of MM1 (or get things right for that instance

). Even in the offchance it happened they still wouldn't have any reasons to prefer Mystrock over Marine now.
9) Salome - Another character which I would deem next to impossible to turn as she is deeply grateful to Marine.
10) Wolfgang - Next to nothing is known about him, as he didn't have enough lines in MM4. While not much suggest him being a spy (I am a bit troubled by Borgon pausing before describing him as another kind of mercenary), he does have the weakest bonds with the rest of the group. If one of the 13 proves to be a traitor and Benoit isn't in love with Marine, then my money is on Wolfgang. Hell, my money is on Wolfgang either way.
11) Bokden - As I already explained I highly doubt him being a spy. I also see little chance of him becoming one, since his agreeing to help Marine means he at least likes her (have I mentioned chemistry yet? Maybe I should do so again).
12) The Bear - Although he hates Marine, he absolutely loathes Mystrock, so I'd say we're safe there.
13) Suzuki - She's his granddaughter. The only way I can see him betraying her is if he goes through a bout of dementia.