Numbuh 274 was introduced in Operation: C.H.A.D., in which a pair of villains known as Mega Mom And Destructo Dad begin neutralizing KND operatives all over the world until Chad and Sector V are the only active KND agents left on the planet after the rampage. To his shock, the villains are revealed to be Numbuh 274's parents, who were eliminating all the other KND members to ensure that their son remained the best. Chad stopped his parent's insane endeavors by threatening to get a tattoo. He later assisted in the overseeing of the (failed) decommissioning of Numbuh 206.
In Operation: T.R.E.A.T.Y., it turns out that Numbuh 274 is still KND operative, undercover as a Teen Ninja. Practically every mission Sector V has ever performed was being secretly aided by him. Proving that he was never actually a villain. However, his hatred of Numbuh 1 was real and the only villainous things he ever did was to get revenge on him for ruining his life and kill him off so he can be chosen to be the Galactic Kids Next Door's earth representative. Meaning that his roles as a villain in the previous episodes was only to make Numbuh 1 suffer and prevent the rest of Sector V from interfering with his plans of getting revenge on their leader.
In the GKND prologue, Chad is captured by the Galactic Kids Next Door and imprisoned (presumably by Numbuh Infinity) for almost exposing their secrets and whereabouts to Sector V. While chained up, Chad tried to plea with Numbuh 1 not to go along with the GKND's twisted plans and ideals on how to deal with adults as he pointed out that the GKND will not only bring destruction and genocide to all adults on Earth, but for the rest of the Universe.
Chad knows the Galactic Kids Next Door are pure evil and they are a whole lot worse than the adult tyrants on Earth. Chad is then taken away by the GKND before he say anything else to Numbuh 1.