Significance of Ethan's Batman Shirt

Ethan's Batman shirt, which he wears every day, is a symbol of his connection to his deceased brother, although the meaning is not made clear until the end of the novel and thus functions as a symbol of the mystery surrounding Ethan from Jessie's point of view as well. She wonders why he wears the same shirt although can never muster the courage to ask, and thus is remains an unresolved mystery until he reveals himself to be SN and tells Jessie that it belonged to his brother, Xander.