Grace seems to be this sweet, innocent young women, with some religious beliefs. It's always the least likely that is the most evil and can get the closest to the protagonist. I wouldn't say that she actually commits all the murders or makes all the appearances as RJ but through manipulation and intelligent strategies she controls the RJ killings. With her beauty and insane influence she probably got the killer of the couple making the insane movie to say he was Red John, but really it was her pulling all the strings. Her hair is red and since she probably considers her victoms as whores, she is therefore their "JOHN". She would naturally get as close to Jane as possible since he is her only real competition in game of not getting caught but pushing the envelope as they would say.

I know I know, she is female, but she has a soft voice that naturally sounds like a man. She has red hair!  Plus what kind of sinister evil person would love her job more than a person, NO she needs to stay in the department to keep tabs on Jane.  Plus I believe she has been in the department for 9 years, same as how long red john is on killing spree.  She could get the inside info right then and there.  The one time Red John saved Jane he had called Lisbin and told her where he would be, thus only lisbin and maybe the team knew where he would be.  It is quite unlikely that a serial killer would keep tabs on Jane that often.  Maybe its coincidence but I think not. Oh and did I mention, She was "seeing" a guy that killed people when Jane was blind for that one episode, but he didnt shoot her and she didnt appear in any distress.  And now she dates a FBI agent for more inside info.  Lets face it she is the best with the computers so she could easily hack the ones at the CBI to send all those messages....

Grace in my opinion is the most reliable for the role of Red John, the reason that leads me to think that she is the killer is that first part of the CBI, so it has at hand all blown from the district. It was also the girlfriend with a follower of Red John, never close feeing great with the team to part with Wayne and see if the end of the third season has killed her boyfriend when it was discovered

I’m from Uruguay and my English is not very good but I’ll try: Since season 1, the character’s development of Van Pelt seemed suspicious to me. I don’t think she is Red John because I believe he’s a man and older than Grace. But RJ could be someone related to her…
Reasons:
Just in the first episode of the series there are a lot of hints about Van Pelt but attention was directed only to creeps Brett Partridge and Dr Wagner.
- Van Pelt is the only one in the team who for sure arrives after Patrick. She’s the new one.
- Those gestures she makes when she wants to look like a good and sweet school girl. At the beginning seemed hers but now she uses them when she wants to manipulate someone. 
- How her face changes when she gets angry with Jane. Her comments telling him: Poor sad man, etc. Jane told RJ “sad, sad man”.
- Her dogmatic beliefs in psychics (cousin Yolanda), Red John wrote to Jane: “If you were a real psychic…”, he also believes in them.
-The comment Jane made to Wagner: Why magicians have beautiful assistants? To distract attention! Van Pelt is red haired and beautiful. She’s distracting everyone.
-How could Jane know her father was football coach just from her demeanor? Suspicious… maybe he knows her, recognizes her. Also, Jane told her she believes in Yolanda because she let her talk with someone who died and VPelt loves. (Her sister who committed suicide…, maybe because of Jane?)
Well, now I have to go but i'll come back with more reasons.

I think they mean Red John is VanPelt struck by multiple personality disorder. Which means she would change her identity and behavior patterns unknowingly because multiple personality accompanies memory loss so that one personality would not recall anything done by the other. Basically it's two personalities taking over the same body. in this case Grace and Red John are two personalities of the same person!
I can see why you say it but I have doubts specially because of the time frame! VanPelt looks to be in her late twenties when Red John's career started in 1998!

Actually, I think Red John is Grace Van Pelt.
First of all, it is more than obvious, that it is someone from inside. I am pointing at the team. Think about it! She's obviously smart, she seems to have a "moral agenda". - Red John has been starring at us from the beginning. Plus, none of the characters, backgrounds have been developed. The killer's trademark is a crying-smiling face. That would hint to someone obvious! If I remember correctly, in an episode Patrick, suggests that Wayne is going to invite Grace, to sleep with him. Afterwards Patrick said, Grace already prepared an answer. My question is, how did she figure out Wayne's intention - she did, because she is a mentalist.
Think about Red John's modus operandi. At first, you see the "art" and afterwards, the murder. I think we are seeing the crying-smiling face at the moment. We haven't looked at the actual murder.