Out of the two theories I believe of who is the truest believer, I believe that it is more likely to be Henry instead of Tamara. When I think of the word believer, I think Henry more of Tamara. Also, I think that the reason why Peter Pan is coming after Henry is because of the fact that he wants Henry’s heart which is probably why the episode is called The Heart of the Truest Believer.
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Keeper of Tamara's death, Cyrus's bottle, Ariel's red hair, and the mermaid that got turned into a statue in the season 3 premiere episode.