Eli is the name of a background character, a rotund boy with short hair who was named when he was interviewed on What's Your Favorite Word? when he was 10 years old, at which point his favorite word was "monkey".
He appears in Theme Park WHAMpage when he and Sandy are being bounced by the Whammer and WordGirl intervenes to save them. He has a speaking role here though, and seems to have been enjoying the Whammer's bouncing.
Interview for WYFW
My favorite word is "monkey". I am just a person who likes monkeys, and I just like the way the word sounds. Mon-key! It just sounds good.