Reuben was overweight in middle school, but went to a fat camp and boarding school for two years. Reuben was bullied and teased most of his life, because of his weight.
He finally made the decision go to a fat camp. When he lost all his weight, he is like a new different person, people treat him differently and he looks differently too. He is little more sociable now since he is more confident. So when he goes to high school he is really scared, because he finds out that Carson, the main guy who used to torment him is Mr. Popular and his in mostly all his classes.
Reuben tries to avoid him as much as possible, but it seems Mr.Popular takes a liking to him. So basically, Mr.Popular doesn't think about the kid he used to torment, but Reuben will always remember the guy who made him want kill himself at some point in his life.