Red John is Det.Marco Francis. 
1.) He has had only one appearance and that was in the episode 'Seeing Red'. Title is self explanatory. 
2.) He is a police officer with access to it's networks and being a detective he will obviously meet damaged people, damaged people who he can use, like Lorelei after her sister was killed. 
3.) The message written on the wall in Mexico 'He is Mar'. Very possible that he was writing 'He is Mar(co Francis)'.
4.) Det. Francis even fits the description of Red John. He is a lot like Jane, charming, manipulating...
5.) He and Jane shaked hands only once!