Topper is one of Digger's children. He is Filler's brother and, as he states in Trouble in Zombie-Town, his job is to top the bottles. He is killed in Gameknight's alternate timeline.
Topper craves to be treated like an adult, as he tells Gameknight in Battle on the Ocean Shore, and so Gameknight tells him to do the best he can to help the army. Topper, though now acting older, still loves to play and looks up to Gameknight. BTW, In the 3 last books in the series, it says Topper is a Girl.