Negative Numbuh 5 is Abigail's dorky evil counterpart who wears braces and glasses, appears to have buck teeth, and hates lollies. She carries a S.P.L.A.N.K.E.R. that looks like it's made of metal, and instead of a magnet, there is a spring. When she suggests they attack positive Wallabee, she says it with a lisp.

Negative Numbuh 5 is physically described as having a blue shirt on with white stripes going down the arms, red shorts and white shoes. Negative Numbuh 5 also wears a bright red hat.