Who Is Red John?

We don't see anything from his point of view. The writers don't give us the chance to analyze him from the actions he is about to commit, nor giving us the present of his monologues. He is hidden well. What we know about him is what Jane and his team uncovers.

Some made distinctions regarding him as a womanizer and a perverted serial killer as two separate persons. Not in my opinion. He collects woman like scalps on display. The difference is in those who fall for him and those who don't. The ones that accept him as their leader/lover live to see another day, those who don't... well the smiley is upon them.

Following this logic with Lisbon in mind, to whom did she denied her love?
Haffner and Marcus.
Haffner is out of the picture at the time, while Marcus I presume is still in it. At least he is still alive. Will he let Lisbon go to the arms of another man or will he punish her severely? This question is worth of a Red John answer.

The fake letter Jane wrote in Bluebird that he graciously "let" Lisbon the honor of cracking it might have been Jane's way of inconspicuously telling her "stay, don't go. H is M", but clearly Lisbon didn't get the message so he did the only thing Jane could have done to keep her from going into the lion's den.

Whether we like the result or not one thing no one can deny to the show, is that there is no lack of suspects. It almost feels like, whenever the creators cast a member they gave them the script with a side-note:
"Top Secret for your eyes only; Here is your script. BTW on the end YOU are THE Red John. Now act like him!" (Yes, female characters and extras included)