Numbuh 42 is a member of Sector X, a very lazy operative who seems to be more interested in Yipper cards than anything else. He is a friend of Numbuh 2 and his provider of trading cards. Since Sector X has few missions, Numbuh 42 has a lot of free time to collect cards to trade with his friend. In Operation: U.N.C.O.O.L., he stole a Yipper card from nerds, causing them to turn into Nerd Zombies, kidnap his teammates Numbuh 34, Numbuh 78, invade the Sector X Treehouse, and almost kidnap every member of Sector V except for Numbuh 2 before he finally agreed to give the card back. The Nerd Zombies also required another card to trade for the hostages, which he is heavily implied to have stolen from a Nerd Werewolf.
He also appeared in Operation: A.M.I.S.H. at the big 2x4 Technology fair, where he invented a fast-growing treehouse seed. He also mentions in this episode that he's a fan of the fictional science-fiction TV show Doctor Time Space in the Continuums.
Considering his nerdy persona and fascination with science-fiction Numbuh 42's codename is most likely a reference to legendary science-fiction comedy book series The Hitchhiker's Guide to the Galaxy, in which the ultimate answer to life, the universe, and everything is said to be the number 42.