i think RJ is him, bret stiles because of some reasons
1st. they have ever met before and shoke hand each other
2nd. they have same caracter on manipulating and lying.
3rd. stiles knows RJ so well, he knows what RJ plan (season 1)
4th. stiles has  very large connection (exmple he can help PJ to release lorelei and so many). RJ must have large connection
5th. RJ must have a relation with visualize as we known (from season 5)
6th. RJ dont want to kill PJ by some reason (such as RJ feels that they could be come friend), we know that stile also dont want to kill jane because of the same reasons...

it's my theory, but we still dont know...