brianna is Nikki's little sister who is scared of the tooth fairy and is described in the book as, "bratty"
She is 6 years old and thinks that Brandon has the cooties and can't go to the bathroom because she is afraid that the tooth fairy is hiding in there. So often wakes Nikki up in the middle of the night to go to the bathroom, and if Nikki isn't quick enough, Brianna will go to the bathroom on the floor. She hates ballet, and seems to like drawing just as much as Nikki. She also has a puppit named miss penolope, who is really only a face drawn on Brianna's thumb and index finger. She blames whatever she gets in trouble for, on miss Penolope.