So the three people that Peter specifically links as the First People; Walter, Astrid, and Ella happen to be the three witnesses of the "Brown Betty" tale. A tale or any fiction is like a creation of an alternate reality (we've seen how much of Brown Betty has "come to reality" in the show).
Walter being the storyteller is creating that imaginative reality, and Peter said "you, most specifically" to Walter being a First Person. Ella and Astrid were the two witnesses to Walter's tale (pictured above).
Ella even partakes in this process of creation (storytelling) by changing Walter's original ending to his tale, into a "proper" ending. This revised ending results in the important symbolism of Peter splitting his heart to save both himself and Walter; a proposed solution to save both worlds (pictured below). Additionally, in a call-back to BB, the grown Ella in the finale says there is "no happy ending".
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