Cinnamon is Stargirl's pet rat. Yep, rat. So why are we talking about him in the character section? Believe it or not, this rat plays quite the role as events unfold in Stargirl.
First of all, the fact that Stargirl has a pet rat immediately tells us that Stargirl is not your average, well, girl. Let's face it: rats are not the most desirable pets in the world. Even a mouse is an itsy-bitsy more preferable. But a rat? No thanks. Stargirl, though, is all about rats. She loves Cinnamon, which shows just how much Stargirl sees the beauty in everything.
Leo tells us that Stargirl's rise to popularity is partly due to a backlash against Hillari Kimble, who so cruelly dangles little Cinnamon by his tail over the banister at school. All the kids who witness this cruel act beg her to stop, but Hillari won't. Her cruelty garners Stargirl a large dose of sympathy, and then, at the height of her popularity, a bunch of students follow her lead and buy pet rats, too.
When Stargirl catches Leo stalking her house, and goes out to talk to him, Cinnamon steps in and saves the awkward situation by scampering over to Leo, who is hiding behind the car. When Leo lets Cinnamon crawl all over him, and even root around in his ear, the ice between Leo and Stargirl begins to break.
When Stargirl takes Leo to her enchanted place and tries to teach him about becoming "nothing" in order to hear what the earth is communicating, Cinnamon comes along, too.
Leo doesn't have much luck trying to "erase" himself the way Stargirl explains, so he peeks, and while he is peeking, he sees Cinnamon escaping from his bag. To make sure that Cinnamon isn't eaten by some other desert animal, Leo picks him up and holds him while trying to meditate again.
Here's the cool part: Leo notices that even though he isn't completely successful in his attempts to become nothing, he stops feeling Cinnamon in his hands after a while. In a way, he becomes "connected" to Cinnamon. Now he's one step closer to being connected with everything.