Character Analysis
Sym bases her imaginary friend, Titus, on a real-life hero who died during a grueling expedition in Antarctica (source). That Titus, the real one, was an Army officer who was hired to watch the ponies during the difficult trek. Needless to say, that expedition ended badly. He died in 1911, more than 100 years before the events in the novel, which are set in the present day.
The other Titus, Sym's imaginary friend, came into Sym's life the day her father died. Though she knows he isn't real, he's been good company for the fourteen-year-old, who's lonely and sad. He fills a lot of gaps in her life: He's a father figure and her leading man, as well as her proxy. "He is everything I needed and couldn't find in real life. Of course he is. That's why I invented him" (19.84-86), Sym says.
Titus's sides of their "conversations" consist of all the things Sym knows about him from reading books, plus stuff that Sym herself notices but isn't quite ready to process. "Our thoughts nestle against each other, like pigeons on a wire" (3.52), she tells us. "Everything Titus ever said to me could only have come from inside me. Things I've read. Things I've imagined" (23.48). But when he saves her life in a blizzard, Titus tells Sym something she doesn't know: His body has been dumped in the sea. Sym's assumed he was entombed in ice.
Sym herself cannot seem to account for this discrepancy, so the book leaves it as an open question. Was the Titus who helped Sym her imaginary friend? Or was he the real one, somehow back from the dead? "How could he possibly have told me something I truly didn't know?" Sym wonders. "Oh, Titus! Tell me what it means! Tell me what to make of it! Tell me what to think?" (23.51) But on this question, at least, Titus remains silent.
Fun fact: Titus is a symbol, too, so hop on over to the "Symbols" section to read more about Sym's leading love interest.