why is everyone soooo certain that it is suddenly Summer???

The clue is so far that jane and red john shook hands and they've met but didn't become best friends ! the writers put the pilot episode in season 5 of the series and if you look again you'll see in that story jane shook hands with the JUDGE and when lisbon and jane investigate him he says if you want to hurt a man don't kill him kill his family !! and right after that he asks jane if he has children!!
this maybe be a possibility!!!
p.s Kirkland is for sure an accomplice and Alexa is that friend of red john's in the FBI.

Red Jhon is Agent Kirkland 
Lorelei Martins said that Jane and Res Jhon are very similar and they should be friend since when they shaked their hands
At the end of the episode 7 of the season 5 ( the episode before ) the Agent Kirkland shaked the hand of Jane after hurt him on the CBI office. Jane said he did not know him but he said he know Jane......

I think that Red John is Beuno Heller.

Who is Red John? Keyword being 'who', perhaps it is a 'they' think about how easy it would be. If Red John was more of a company or group rather then person. Think about it, (I'm not good with names) But Jane shot a guy at the end of season 3, the man revealed what Jane's daughter smelled like, but it was later proven he wasn't Red John.. I think in season 2 the man on fire if i remember correctly, tells Jane the same poem 'Red John' Told him. It would seem that Red John is an organization of 20-30 serial killers. All the killings would make it very hard to think ONE man or woman did them.

I think Ellis Mars is Red John. Just a hunch. He played a serial killer in Cold Case ,i.e., George Marks.  He is under 6 ft (5' 10"). He is mar (if that was a real clue) could also be read as IIe is mar...which could be a reference to his true identity. Ellis Mars seems stupid but Lorelei says RJ is just like Jane. PJ deceived RJ in Season 4 finale...So RJ could haave done the same thing to PJ too.

Because the Red John Story is practically the entire plot of the series, the identity of Red John will not be revealed until the very end. So, all we know for now is who Red John is not.

1) Red John is NOT Alex Jane (Patrick's father). In Red John's note to Patrick, he accuses him of being a "dirty money-grubbing fraud" and a "dishonest little worm". In the episode Throwing Fire, we learn that it was Alex Jane himself who made his son a dirty money-grubbing fraud.