I think its Brett as well, also look at the episodes where people tend to mimic red john's work. Patridge appears in those particular episodes nagging at jane.

Brett Partridge -the character and the name are suspicious.
If he’s not RJ, than he’s an associate. His character has too many lines and
too much overt personality for someone not integral. That jumped out at me right
away. In addition to the previously mentioned reasons by the other posters
concerning BP, I like the William Blake connection. A Brace of Partrdge is a
painting by WB of two partridges lying on the ground together. Maybe dead.
Unclear. Also, the Tyger Tyger poem by WB in 0223, asks the question of who
makes evil (the tyger) and how could the creator of it allow it to grow? A line
in the poem, “Did he who made the Lamb make thee?” reflects RJ’s fascination
with good, with Patrick. But these obsessions are always tied to emotion. What
is up with the real RJ? What is this deep pain? It’s not random killing. There is
symbolism in everything he does (I bet). In 0123 Emma is killed and the sister
Maya spared. Emma is the dark and moody one. Why spare one? We are to think it
was payment to Hardy, but we all know that RJ could’ve done it himself and killed both girls.  We know that. That always struck me as odd.
Why would he need this guy to get her for him. RJ could’ve killed Hardy after
Hardy stole the girs for that matter. So no, I think he kept one alive on
purpose. His obsession with siblings’ love. Make the live one suffer loss. Like
he did. ?? In 0508, we learn RJ killed Laurali's beloved sister. He spared Laurali. So RJ and PJ are brothers? My only
problem with the Partridge as RJ theory is that in 0508 Laurali says Patrick
and RJ should’ve become best of friends after they first shook hands. I don’t recall BP and PJ ever shaking hands. Partidge is a minion? Maybe RJ is Stiles
after all. William Blake has 3 paintings, The Dance of Albion, Newton, and Angel of Revelation. The character in the paintings is unmistakably our hero. You gotta see it. Uncanny.

Tyger! Tyger! burning bright
In the forests of the night,
What immortal hand or eye
Could frame thy fearful symmetry?

In what distant deeps or skies
Burnt the fire of thine eyes?
On what wings dare he aspire?
What the hand dare sieze the fire?

And what shoulder, & what art.
Could twist the sinews of thy heart?
And when thy heart began to beat,
What dread hand? & what dread feet?

What the hammer? what the chain?
In what furnace was thy brain?
What the anvil? what dread grasp
Dare its deadly terrors clasp?

When the stars threw down their spears,
And watered heaven with their tears,
Did he smile his work to see?
Did he who made the Lamb make thee?

Tyger! Tyger! burning bright
In the forests of the night,
What immortal hand or eye
Dare frame thy fearful symmetry?

William Blake did an oil painting "A Brace of Partridge" (two partidge tied together)
Also a Poem called The Lamb


In the the pilot script the first thing Brett Parrtidge says is "Red John enters here..." .
In the very first episode they give you Red John.
Patrick Jane finds him Eirksome gives him the eirks! No one gives Jane an instinctive chill He doesn't beleive in any supernatural thinking. (Or so he says)

So you have the poem The Tyger. Thy fearful symetry.
Brett is Patrick Janes twin brother, like Moriaty to Sherlock Holmes. Wonders how Jane can be so beautiful and good (ish).

Did he smile his work to see?
Did he who made the Lamb make thee?

Red Johns smile a dig at Patricks smile, so adored. Patrick is the lamb, Red John the Tyger.

Patrick Jane says to Brett Partidge when they first meet.."you enjoy your work too much.."

As Brett works at the CBI he is on the inside. I think on the subject of Red Johns voice , he takes recordings of Janes voice in the interview rooms and maniplulates them, like in the season 4 finale.

He obviously has people on the inside with him. Van Pelt being one, she is not present in the 100th episode, Red Dawn. Only when Partrick Jane starts working for the CBI does she appear. Picked by Rigsby for her beauty fooling others with her gullibilty.
Also Bertrem quotes Blake another follower.

Jane must not be aware of his twin but Partridge has always been aware of Jane.

For a while i thought Jane was RJ. But the " ..fearful symertry is in the twin brother the not the person. As in the two Partidges. As in the Two Poems "The Tyger" and "The Lamb".

Theory by Red Herring





 

It's not brett he's to young what he would've been like 15 when he started killing...

I believe Red John would have to a possible homosexual man (no offence to anyone). Why? Because his regular victims are females and he's killed males only when they show a possible threat to him or his identity. This might sound like a weak theory, but as a student of psychology, I'm forced to analyse the personality of the person who is Red John and why he only kills women as his "regular" victims. 

Brett Partridge does make a good suspect because of his placement in the Forensic Dept,his attitude towards Jane and the RJ case investigations, his voice, his appearence (that matches Rosalind and the little girl's description) and moreover, to support my theory, Mr.Plotnick has potrayed his role in a somewhat feminine body-language, almost homosexual.

Of course, there's also the fact that RJ is a rich, highly influential person, making Partridge innocent (except we don't really know his financial status). 

This also makes me think RJ could have something to do with Brett Stiles's cult "Visualize" and it's recurring appearance. Even though Stiles is quite old to be RJ, it could either be him or one of his disciples. 

And finally, I personally think we can suspect one more person: Mr.Amos Van Pelt (Grace's father). I thought of him because of what that FBI mole said about him when he met grace for the first time, mentioning what a great influence he was for him back in high-school.

So here's my suspect list:

1. Brett Partridge 
2. Brett Stiles or a protege of his.
3. Amos Van Pelt (just a hunch)

I think it's Brett Partridge mosly because he's super creepy. I looks like a psycho killer. AND What are the odds that he'd end up working on 2 fake RJ crime scenes...? He and Jane also seem to really hate eachother, that could very well be that they're just faking they're hatred towards one another...

Another complete and very possible theory on Brett Partridge.

Many clues and reasoning always
brings me back to Partridge.
First I thought that perhaps it He is mar
means that is injured, in the sense that Jared does not know who he
is, but have struggled, as happened with Carter Peak and was
scratched, RJ was forced to flee because someone was coming, but
thought he was dead , but before dying he wrote on the wall. Then he
took away samples of trace or altered after they are collected by
Scientific. RJ had to do before the CBI. Or does it mean Marshall in
the sense of police officer.
If RJ is Partridge knew well how to
eliminate all traces of samples of the hospital, because his
fingerprints and his DNA is in the database, such as those of all
members of the police and army. The elements against Brett are the
following:
- The same voice in 2×23 (the voice is very similar, but maybe isn't the same
also in 4×24, and similar at the voice of Carter). He has a particular way of pronunce some letter and some word, and a particular way of speak some phrases
- Its size and appearance fits on Roy, as described by
Rosalind)
- The clothing of her first appearance resembles that of
Roy Tagliaferro, and the masked man who save Jane in copycats scene
- His necktie and jacket with the word forensics,
it seems that the mysterious figure who follows Rebecca in 2×08
-
Admiration for RJ
- Presence on the two scene of copycats (Wagner and snuff movie directors)
- The
picture of Blake  “A brace of partridge”, linked by a Red John poetry in 2x23, The Tyger
- The character
is completely useless for the purposes of the plot, in the second
appearance; others minors characters have an utility in the storyline
- Although this has a name and a surname instead of
simply called CBI Brett or CSI Brett (unlike CBI Ron, CBI Karl, Security guard Jim or other
agents and characters), appears in the pilot (is also described in the script), but
no one ever mentions him. This is a sign that is important. It's only character very minor that he has a complete name
- In 2x23: the
scene between RJ and Jane (goes away and comes back) is the perfect mirror
image of the previous one between Jane and Brett (for me is very important clue)
- The character
sarcastic and childish (RJ kills for fun and fun and getting offended
easily, as the Joker, even if it’s Jane Moriarty)
- He hates
Jane: perhaps you feel nervous and inadequate in its true form, but
through his role as RJ instead became the charismatic and confident,
like many people with dual identities
- When asked, no “He
interpreted physically RJ?”, But “has lent her voice to RJ
MASKED?” Plotnick said, “I can not say anything to avoid the risk
of ruining the surprise,” he could say or explain: “I only
voiced” or “no, I know nothing” but not said, perhaps because
it had a contractual clause which requires them to remain vague.
-
Has an important position (CSI supervisor, says about: “My guys
have to work”)
- For the same age as (1968) could be both a
friend of Tanner, both of Johnson, O’Laughin and Montero (the
perpetrator of the museum) and Kristina
- Hightower is trapped by
forensics (O’Laughlin killed Montero, but also because he hadn't access to evidence)
-
The CSI agents probably is under him (the strange nervous behavior) also work at
the scene of Timothy Carter
- We know nothing of his true
abilities nor his financial situation (maybe it’s full rich of family or
earn easily with scams and casino)
. The behavior of Partridge does not exclude the fact that it can be
extremely intelligent, but wants to hide appearing stupid or
insignificant (Jane says: “the viewers watch the girl assistant,
while the magician does his tricks,” which could be, in the case of
RJ : look at all the famous actors McDowell, Currie Graham and
Bradley Whitford, and should look Plotnick). However, Partridge is a
person who has the power and capabilities, otherwise it would have
been the supervisor of the CSI, as suggested. All these false trails
laid there for detecting, after almost seemed to shout that he was in
RJ 2×23, the answer to Jack Plotnick (which still does not know when
he will return). the evidence and the arguments make it suspect.



Age: Partridge should actually be older
than Jane (at least the actor is older than one year).

If Heller or other writers in future will says who Red John is a
guest and he don't appear for many time in the show, with his face,
Partridge can be return a very strong suspect. The episode 2x23 screams
to us that Red John is Brett.
He's a ghoul, nervous and seemingly uncharismatic, with a mad face, but even Jim Moriarty of BBC Sherlock has a crazy appearance,  the first time we saw him (in the laboratory) looked like a guy with no power and no leader's character, in fact psychopaths know perfectly disguise, like as is also shown in The Mentalist many times.
Then Brett is right in suspect's list.