To me, the writers of 'The Mentalist' wish to succeed in one task, when Red John's identity is revealed - they wish to cause ripples of shock within the audience. Take a look at 'Broadchurch', the killer was someone close to the detectives and was someone whom went under the radar during the investigation. This in 'The Mentalist' terms would be someone who is on the team, Grace is the most likeliest Now I am a few weeks behind than the US so I don't know about the last two episode of this current season. The first episode (If I remember correctly) she is starts at the same time as Patrick and shakes hands with him (that's the clue given by Lorelei). This creates a certain connection to him and if she is/a connection to Red John than she is able to keep up on the investigations of her/or person in connection killings. She often talks about her Father, a football coach, now her father would be the stereotypical age that Red John would be so perhaps he is and she is in connection as 'watchdog'. She has a trauma  in her past which we know nothing about- so this adds a mysterious dynamic to her. She often challenges Patrick's psychologically 'tricks' which highlights the difference between them. If I remember correctly, she had a fiancee who he ended being something to do with Red John. I can't really remember about him but perhaps he did not know about Red John's true identity. She recently got back together with Wayne, this creates even more shock value as it highlights and reminds the audience of their relationship and will create more shock wave ripples. Also her name, it seems forced to me. Grace Van Pelt. The Van Pelt part as the same letters as Red John, the writers don't always choose a random name, the Van Pelt part seems deliberate and forced to imply a connection. So,all in all, she has something to hide, the writers have 'red herrings' (ie. to divert the attention of the audience) so for example that creepy FBI investigator  they are all 'red herrings' and the true Red John is secretly lurking in the CBI department. She could easily be Red John or is acting as 'watchdog' for her secret football coach Father...