- "Well I'll show you, what the numbuh ZERO can do!"
– Numbuh 0
Montgomery "Monty" Uno is the father of Nigel, son of Grandfather, brother of Benedict and uncle of the Delightful Children From Down the Lane. In Operation: Z.E.R.O., he is revealed to be Numbuh 0, who was the founder of the Seventh Age of the KND Organization. It was him who destroyed the greatest super-villain the world has ever known and created freedom for kids worldwide. Numbuh 0 soon became a myth and legend as the years passed.
Monty Uno was the elder son of Grandfather, the adult ruler of the world who forced all the children (his included) to work as slaves in his tapioca factories. One day, Monty refused to be oppressed any longer and ran away. As his brother Benedict attempted to reason with Monty about this in a tree, he accidentally discovered a secret chamber in the tree of coolness containing the legendary fabled Book of KND. Upon reading it, Monty was given the inspiration to fight back against his oppressor.
However, his cowardly brother, Benedict wanted no part in this, believing they had "zero" chances. As such, Monty then called himself "Numbuh 0," leading a rebellion to destroy all the tapioca factories. Eventually, Grandfather appears to personally confront Numbuh Zero, giving him three seconds to go to his room or else. Before he reaches "zero", Numbuh 0 cries out his namesake number and unleashes a device on Grandfather that seemingly destroys him. (It is later revealed by Father that Numbuh 0 had used a decommissioning device on Grandfather to take away his powers.) Thus, Numbuh 0 brought about the Seventh Age of the Kids Next Door.
After growing up and falling into legend, Monty Uno married Mrs. Uno, who was the first female operative in the new age of KND, known as Numbuh 999 (herself), and had a son named "Nigel", who later became Numbuh 1 (himself). His house was built near the tree that housed the Book of KND, which would also become Sector V's treehouse.
When Grandfather was recommissioned by Father, Numbuh 1 attempted to find and recommission Numbuh 0. Upon learning that the legendary operative was none other than his own father, he eagerly recommissioned him. In contrast to his decommissioned self, Numbuh 0 is anything but oblivious or unhelpful, revealing to Numbuh One the location of the Book of KND, Father's identity as his uncle, and the fact that the Delightful Children from Down the Lane were once the missing agents of Sector Z.
After Numbuh 0 reconciled with his younger, scared-turned-evil brother and convinced him to help him defeat Grandfather, the two went outside to face their father, who had by this point turned everyone else in the world into Senior Citizombies. However, Benedict chickens out once again and Monty is left to face his father's wrath alone.
Monty was able to resist the Senior Zombification process by having the Book of KND, which Grandfather wanted to destroy, in his back pocket, buying Numbuh 1 and the rest of Sector V enough time to crash the KND Moonbase into the Earth where they stood, decommissioning them both.
Nigel then attempted to recommission his father once again, but the Recommissioning Module fell apart in his hands, revealing a message to him from Monty telling him that he can never rejoin the Kids Next Door, being an adult, and that as an adult it is his duty to be a good father to Nigel. He also wanted Numbuh One to fill his page in the Book of KND with some "crackerjack stories" for future generations. After the message ended, a now "re-decommissioned" Monty asks Numbuh One if he and his friends were playing with "2x4 techno-bob-thingies". Numbuh One said, with tears in his eyes, "No, Dad. I was just playing with an old friend." and he embraced his father, receiving a hug from Monty. Later on, the KND praised Numbuh 0 as a hero and christened a massive golden statue of him at the new Moonbase.
Numbuh 0 and his wife were temporarily recommissioned during Operation: I.N.T.E.R.V.I.E.W.S. to congratulate their son for winning the scavenger hunt and to say goodbye, since Numbuh 1 is joining the Galactic KND and won't be allowed to return to Earth. His wife warned Numbuh 1 to stay away from all those dangerous black holes and supernovas in outer space.
- Great Grandmother: The Annoyingly Cute Triplets Who Lived Upon the Hill's Mother
- Grandmother: One of The Annoyingly Cute Triplets Who Lived Upon the Hill
- Father: Grandfather
- Younger Brother: Benedict Uno/Father
- Wife: Mrs. Uno/Numbuh 999
- Son: Nigel Uno/Numbuh 1
- Adoptive Nieces & Nephews: Delightful Children From Down The Lane
- Niece: Numbuh 10
- Numbuh 0 has tasted the "fourth flavor" since he said after being recommissioned "I have a hankering for some blurpleberry ice-cream - it's the closest thing to the fourth flavor I've ever tasted!"
- Numbuh 0's brother, notably the villain Father, was probably named after Benedict Arnold, both being traitors.
- Despite appearing in Operation: Z.E.R.O., he is the only character who doesn't make a background appearance in The Grim Adventures of the KND.
- It is revealed in Operation: O.O.M.P.P.A.H. that he hates the tuba but loves the sousaphone.
- His voice is an impression of Allan Melvin.
- Numbuh 0 may also have a Negative counterpart in the Negaverse; however, it is likely both counterparts are the same personality due to how the number 0 is neutral between the positive and negative numbers.
- If "NUMBUH 0" is entered in the Rainbow Monkeys Website, the text returns, "unknown." This is a reference to Numbuh 0 being a myth.
- The same case happens with Numbuh 999.
- He picked up learning how to play sousaphone as a child.
- Monty only realized that the Delightful Children were his nieces and nephews once he was recommissioned.
- Since decommissioning erases all memories of the KND, it's likely that once he decommissioned himself Monty does not remember he has nieces and nephews.
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