Conversational and Confessional
Regan is the narrator in Luna, and so the writing style comes across as pretty conversational and even confessional, even though Regan doesn't really open up to other people in normal life. In the book though, she reveals everything that happens, giving us the run-down of dialogue and events so that we are kept in the know with all the events that transpire. She also gives us confessional tidbits where she reveals her own emotions about everything that's happening. For example, when Luna shows up at school, Regan has the following thoughts:
How could Luna show up at school? People knew we were related. Chris knew. Liam and I would never be disassociated now. Me and Luna. They'd always see me as Regan—the one with the transgender brother. (23.3)
She uses these interludes in order to give us more emotional insight into how Luna's transition is affecting her emotionally. The novel also uses flashbacks regularly to give the readers insight into Luna and Regan's upbringing. For example, when Regan recalls the one time she had her own best friend, she flashes back to a sleepover she had where Luna slipped out of his male role:
I think Carmen suspects. I don't know what she suspects, exactly. Maybe that he's gay. He isn't gay. If he were, I'd probably tell her despite the fact I'm sworn to secrecy. (5.183)
These flashback segments are used quite liberally to show that Luna's transgender identify isn't a new thing—she's always been a girl. And this lets readers know that being trans is something Luna can't shift.