Thomas "Tommy" Gilligan is the 7 year old younger brother of Hoagie Gilligan (Numbuh 2).
In his first appearance, ''Operation: T.O.M.M.Y., Tommy is unable to play with his brother due to him being occupied with the Kids Next Door, and out of jealousy forms an alliance with the Common Cold to capture his brother's teammates. However, the Common Cold betrays Tommy and captures him as well. It was only through the intervention of their mother, Betty Gilligan, that the Common Cold was defeated and the operatives were freed. He has a fedora, gloves and large cape.
In the Season 8 episode Operation: D.E.C.O.M.M.I.S.S.I.O.N.E.D., right after he graduates from his Kids Next Door operative training program, he decides not to be Numbuh T anymore, he decides to become Numbuh 987,988.