Mr. Syme is Pony's English teacher. Pony likes him and says that he's a good teacher, and really cares about his students.
He's also the one who encourages Pony to improve his grades and his life by writing about his experiences:
"Anything you think is important enough to write about. And this isn't a reference theme [research paper]; I want your own ideas and your own experiences" (12.10).
So, in a pretty major way, Mr. Syme is responsible for this novel. Without his encouragement and all-around awesomeness, Ponyboy might never have set pen to paper and told his story.