I believe it may be either Brian Kendrick or Stevie Richards.
Brian Kendrick- He just ups and joins EV2 out of nowhere before "They" were revealed and could just be another phase in Immortal's plot to fully gain control of TNA and rub anyone out that stands in there way. Think about it...they all pretty much know him, Foley gave his blessing for him to join which makes it easier for everyone else to trust him. He gets close enough to all of them to know how to cause friction in the group and tells Eric Bischoff just how to do it. There appears to be some clues...how he seem to hang on Mick Foley's every word at his book signing and the fact that he's been missing for several weeks now during RVD's bouts of paranoia. Perhaps, to eliminate himself as a suspect.
Stevie Richards- This is just based upon the statements he made on ReACTION last night. He stated that he feels that he is the strongest member of EV2...which was a really random statement and seem to insinuate that he maybe somewhat jealous of RVD and/or Tommy Dreamer. This may go back to his constant encounters with Abyss during the beginning of the EV2 storyline. Perhaps he may feel that he was the only one who had the balls to confront Abyss alone but never recieved the proper credit for it, while RVD on the other had gets rip to shreds by Janice but everyone still focuses all their attention him.