Grace seems a plausible suspect because:
There was a hint (I think in series 2) that she had a traumatic past, but we have not found out much else other than that she really gets touchy on the subject of suicide (she snaps at Rigsby when he makes light of a woman who has apparently jumped of a bridge to her death).
Also, why have we been given more insight and back -story into the other 'central' CBI agents (Theresa, Cho and Rigsby)? For example we learn about Theresa's closeness to her brothers and the affect her alcoholic father had on her life; we meet Rigsby's father and learn about his criminal background; Cho has an entire episode dedicated to his ex- gang lifestyle, so why is Van Pelt's background information limited to the fact that we know her dad was a competitive football coach and she seems to have lost her sister?
She has red hair and as we know women with red hair often appear in episodes. She was the last person to join the CBI team, and I think this was revealed to be not long before Patrick Jane joins the CBI, in the episode which flashes back to when he joined the team?
She is also reluctant to transfer to another department when her relationship with Rigsby is made public, and is particularly keen to keep their relationship under wraps in the first place- this may seem sensible because she was last to join and a woman keen not be overlooked- but could it also indicate that she needs to stay close to Jane?
During the most recent season, has the actress been missing due to pregnancy? If so that conveniently explains why Grace was written out to go and complete a computer course to develop her skills and career, but this would also be a convenient amount of time to go elsewhere and develop/do Red John tasks!
Grace is a whiz on the computer, and is often the character who encourages Jane's tricks- much like Red John, who enjoys keeping track on Patrick's activities and seems to me to respect and admire what he does.
I think it would be fantastic in the final episode to go back in time to each key episode and for it to be revealed that Grace's word or actions have had a sinister double meaning/ulterior motive! This would be so dramatic and would really be a terrific example of writing! Unfortunately I don't know how likely this is going to be....
