I think the title refers to Henry, and also to why Peter Pan is looking specifically for him- that Pan is looking for the heart of the truest believer, namely the one who believes most strongly in magic.
I wonder what Peter Pan could possibly want with the heart of the truest believer. Perhaps he needs it to break a possible curse that envelopes NL. Yet, in order for Peter to get this heart, it sounds like Henry’s life would be over.
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"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy