I am sure Kristina was an accomplice of Red John. Aprart from obviosu hints like her red hair, there is one damning clue: when Jane and Kristina go on their *date*, Kristina does a ocld reading on the waiter, saying 'roll-tide' to him. Later, when Red John 'saves' Jane from the imitator-kids, he whispers to him in his ear: Kristina says hi... roll-tide.
This is a clear confession that Kristina's cold reading of the waiter was a set-up, the waiter is a Red John accomplice as well, and they set the whole thing up to confound Jane.
Kristina's last use to Red John and her ultimate sacrifice is to be used and abused by him so as to be transofmred into that catatonic wretch all the better to taunt Jane, who will thus never know she was herself a tool of RJ.