I believe Red John is Rob Kirkland's twin brother, Michael.

1. We have not heard anything of him until now

2. We have been told by Rob that their mother committed suicide due to their father being a drunk which let to his brother becoming a drunk

3. Rob tells us that he tried to help his bother but later on tells us that his twin brother had "help from someone" which could have been Bret Stiles.

4. Bret Stiles tells us that he knows Red John personally which could indicate that Stiles helped Rob's brother which back fired and turned him into a killer?

5. Rob also tells us that he believes Red John killed his brother because he never heard from or seen him after he had help from this person

All angles line up with Michael... He is my est bet of being Red John.