In the very first chapter in the book, Leo Borlocks says that when he was little, his uncle in Pennsylvania had a porcupine necktie, and Leo had thought it was "just about the neatest thing in the world". When Leo's family had moved to Arizona, his uncle had given him the tie, prompting Leo to start a collection of them. Unfortunately, he had never been able to find any more porcupine ties. On his fourteenth birthday, Leo's mother had called in a short feature about Leo to the local paper, and in it, she mentioned that he collected porcupine neckties as a hobby. A few days later, he found an annonymous gift at his front door - a porcupine necktie. It was a couple of years before he actually met the giver of the gift, but even then, Stargirl was watching him.
Stargirl had a habit of learning about people's wants and needs through quiet observation. She then would go out of her way to meet those wants and needs, just for the joy of making someone happy. Her intentions were not always interpreted as she meant them, but her heart was definitely in the right place. It was she who sent Leo a porcupine tie, before he ever knew who she was.
The very last chapter of the book takes place many years after Leo has seen Stargirl for the the last time. When they were in the eleventh grade together, they were friends for awhile, and even started a relationship, but he had been unable to accept her for what she was, and the relationship lasted only a short while. No one ever knew where Stargirl went when she disappeared from the area with her family, but Leo knows that somehow, she is still watching over him. As proof, he notes that last month, a day before his birthday, he had received an annonymous gift in the mail - a porcupine necktie. Somewhere, Stargirl still remembers him, and is looking out for his happiness.