I totally agree that Tamara is the likeliest candidate for being a “true believer.” She has faith in her mission to irradiate magic. Just imagine her reaction when she realizes she’s in a magical land ( i.e. Neverland).
ETA: “The Heart of the Truest Believer” could mean that someone else (e.g. Peter Pan) has Tamara’s heart. Perhaps she’s being controlled to do everything.
My first thought was Tamara actually, then I also thought of Star Trek: Enterprise opening theme song. But I think we could see a contrast between our characters and their beliefs and why they believe the things they do.
I’m not sure how I would feel if Tamara were being controlled by PP. Most of us do not like her and but if she were being controlled by PP, I don’t think some of us could easily forgive Tamara for all that she’s done so far.