The puzzle that Ethan buys for Julian is all wrong—"not half wrong; it was all wrong" ("Ethan".118)—until it's not: ""Before Julian opened up my gift, I knew that it was going to be just right" ("Ethan").
Ethan thinks it's wrong because it's a heart—a pink heart, with a red heart inside it, and the store clerk wraps it in pink paper while another one asks if it's a "present for [his] girlfriend" ("Ethan".117). Yeah, we have to say that it seems a little questionable. But it turns out the puzzle is so right that Julian wants to do it right away, and he even makes more of a puzzle of it, by hiding the last piece until the last minute.
To be honest, this symbol crashes through the book with all the gentleness of an anvil. The puzzle, which all four of them put together, is like their weird, unexpected friendship, and the final, missing piece is Julian's little bit of magic. Puzzles show up all throughout the novel, in different ways: the puzzle of Julian's invitations to tea, the puzzle of figuring out why Mrs. Olinski chose The Souls, and, of course, the story itself—a multi-piece puzzle that requires a little bit of magic from you, the reader, to put it all together.