Reese Worthington, according to backyard sports wiki, is a slightly nerdy boy who is known for having asthma and constantly swallowing his retainer. Kimmy Eckman and Dmitri Petrovich are known to hate him. Episodes focusing on him will have emerald green title cards.
Note: When Kimmy said she hated Reese, apparently, according to wiki, she was just joking around. Just because Kimmy and Dmitri called Reese a nerd doesn't mean they hate him.