Stargirl is a young woman of action. We learn a lot about her kindness through them. In fact, when she is talking about how she comforted the player from the opposing team when he was hurt instead of letting someone else do it, Stargirl tells Leo, "I didn't think. I just did." In most situations, Stargirl knows what she believes is the right and kind thing to do. She doesn't wait to put them through the filter of what society will think of her if she does them. If she sees someone in pain, she comforts him. If she reads about someone who she thinks needs cheering up, she sends an anonymous gift or card. If she knows of a child who is coming home from the hospital, she joins in the celebration. If she knows of someone who died, she joins in the mourning. We get to know her by what she does.
Leo is a man of inaction. He is characterized as cowardly by the things he does not do. For example, he does not come out of hiding when he is caught going by Stargirl's house. Instead, he has a whole conversation with Stargirl from his hiding spot. This foreshadows other cowardly moments of inaction. He doesn't stop the "Hot Seat" show when things become mean-spirited and out of control. He doesn't stick up for her when he hears people say nasty things about her. He doesn't cheer for her on a couple different occasions when he really feels like doing so. Leo is like Stargirl in the way that he knows the right and kind thing to do. However, Leo filters his thoughts through what other people will think, and therefore he does not do what he wants to do.
The main problem for Stargirl is that many people misinterpret her actions. They do not know the reasons behind them, and jump to negative conclusions. For example, they assume she is a traitor because she comforts a player from an opposing team. They assume that she is being nosy when she attends a stranger's funeral. As readers, we learn the reasons behind her actions through her dialogue with Leo. If Spinelli hadn't given us the skinny on their conversations, we wouldn't have understood our girl's motivations. After all, the only voice we hear is Leo's.
Without having the benefit of hearing the stuff Leo thinks and says to himself, we could easily misunderstand his character. If we saw him fling Cinnamon off his shoulder without having the benefit of understanding just how upset he is about the big sign Stargirl posted at school, we would write him off as the world's biggest jerk. (Okay, so in this particular moment, he is being a jerk, but maybe not the world's biggest.) But the handy-dandy first person narration clues us in on Leo's thoughts and opinions. And those thoughts and opinions show us that he is a creative, intuitive, and deep down good person. He is just not mature enough yet to follow through on his thoughts and do the right things when he is under the weight of society's pressure.