Hello, guys.

I still think Red John is Gale Bertram. Some people here have posted theories based on character's names. Ok, let's go:
Gale is a common unisex given name. It is derived from the Hebrew name Abigail, meaning "Happy God; Father in rejoicing; a father's joy; cheerful; joyful." Source: Wikipidia.

Bertram: This interesting surname of some antiquity derives from the Old Germanic personal name Bertram, a compound of the elements "berht", bright or famous, plus "hrabn", raven. The raven was the bird of Odin, King of the gods, in Germanic mythology. Source: https://www.surnamedb.com/Surname/Bertram

But the name it's not so relevant as the personality traits of the character. Bertram is an intelectual person, he quotes "Moby Dick" and a poem by William Blake. And he also plays Pocker (although he acts like he isn't a good player, so Jane helped him). 

In episode The desert rose, 06x01, how could Partidge lock yourself in that wardrobe? I guess there are 2 possibilities for the big clue in this episode:

1. The door, whiche opens up alone. This means there is someone in the house.

2. I almost see some shadow of a spider in the curtin. that means Ray Haffner could be spooked by this spider, since he has aracnophobia. (C'mon, I don't believe it)

I think it's Bertram. Rewatch the episode when Bertram first appears, "Red sky at night" (3x01)...Both Gale Bertram and Michael Gaston deserves this honor.