There are certain parts of The Disreputable History of Frankie Landau-Banks where you have to bite your nails and hold your breath, because Frankie is doing some pretty crazy and subversive stuff. Stealing keys, sneaking into the hidden tunnels at school, putting bras on school statues, following her boyfriend in stealth mode… these are not shenanigans for the faint of heart. And even when Frankie gets caught and her very own boyfriend furiously turns her in to the authorities, she acts with grace and courage. She admits readily to all the things that she did and faces the consequences. 'Atta girl.
Frankie's pranks are not a result of her courage, but a result of her stubbornness. Real courage would mean that she demand that Matthew include her in stuff—to his face, from the beginning.
It takes Frankie a lot of guts to tell Matthew what she's been up to, and she does so because she wants him to finally know whom he's dating.