"The king does not pretend!"
-- King Sandy
King Sandy is a young boy who thinks he is a king and had a crush on Numbuh 3, who he considered his "princess", and later, his "queen". He is served by the "Knights of the Round Towel", who are actually his older cousins. Sandy first appeared in Operation: B.E.A.C.H., residing in an oversized sandcastle. He meets Numbuh 3 when his Knights bring him to his sand castle stating that she was found building a castle of her own on the beach. He is intrigued by her beauty and asks her to be his queen - Numbuh 3, who believes this is all a fun game, says yes, when she doesn't realize he is serious.
Meanwhile, Numbuh 4 is seen telling Numbuhs 2 and 5 about Numbuh 3 being kidnapped and suggests that they get her back. Numbuhs 2 and 5 exchange devious glances and ensue chanting "Wally and Kuki sittin' in a tree, K-I-S-S-I-N-G!" In reaction, Numbuh 4 makes an excuse about Numbuh 3 owing him a quarter, so they must retrieve her.
Back in the castle, King Sandy watches Numbuh 3 as she admires her pretty wedding dress, but all too soon hears word from one of his Knights that someone is attacking the castle. He tells Numbuh 3 to stay inside while he checks it out; he goes outside to a sand balcony and sees Numbuh 4 wearing a bucket for a helmet, shouting at him to give back Numbuh 3. King Sandy taunts him, so then Numbuh 4 gets angry and tells Numbuh 5 to catapult him into the castle wall (being made out of sand, this would not hurt him as much as a real castle wall would). Once on the balcony with the king, they engage in battle with a sand shovel and wooden sword until King Sandy pauses to remark, "You must really love the princess to fight for her so badly." Numbuh 4 snaps and shouts back, while swinging his shovel at him, "She - just - owes - me - a QUARTER!!" Just before he can do Sandy in, the Knights arrive and defend him, leaving the king to go back to Numbuh 3 and lead her up to the tower where they can get married.
As they are running up the sand stairs along the wall of the tower, Numbuh 3 spots Numbuh 4 as he fights and calls out to him. When he looks up, she happily exclaims that she's getting married, which shocks him; he steps back too far and falls over the edge of the balcony and he passes out.
When he finally wakes up, he and Numbuhs 2 and 5 are tied to a sand column in the tower, close enough to witness the "marraige ceremony" being preformed by a surfer-esque lifeguard. Numbuh 4 asks Numbuh 3 is she's really getting married, and she assures him that they're only pretending: King sandy overhears her and shouts his signature line, "The king does not pretend!" He and Numbuh 3 argue about it, her claiming that she's only ten years old, and he replies that he likes older ladies. Shortly after a pause, she still refuses to marry him for real, so he decides to force her to go through with it. Numbuh 4 then breaks free and begins to fight him again - however, the lifeguard asks how long the ceremony will be because he's worried about the tide coming in soon.
Upon learning this information, the kids separate and the sand castle is struck at the base by rough water coming in fast for High Tide. While Numbuhs 2, 3, 4, and 5 escape, the castle collapses and out of the rubble pops up King Sandy and his Knights. He yells after them that he will return, and then his mother is heard off-scene telling him to come back with his cousins. They make their leave with King Sandy still enraged about his failure.
He returned in Operation: K.A.S.T.L.E., his castle this time located "Rainbow Monkey Happy Sugar Land", and tries to force Numbuh 3 to marry him, threatening to dump several Rainbow Monkeys into a pot of boiling nacho cheese. By the end of the episode, Sandy abandons his pursuit of Kuki and falls in love with her sister Mushi.
At the end of Operation: S.P.A.N.K.E.N.S.T.I.N.E., Sandy is seen helping Mushi escape from her room after being grounded. Sandy and Mushi make their final appearance in Operation: I.N.T.E.R.V.I.E.W.S., stealing Professor Triple Extra Large's perfect snowcone during the scavenger hunt.
- Sandy also appears in the game, Rainbow Monkey Sundown on Cartoon Network's website, based on Operation: K.A.S.T.L.E., where you play as Numbuh 3 as she escapes from his castle. Sandy appears as the final boss of the game.
- Sandy is one of many villains in cartoons, live-action TV and movies to use the common trope of choosing female protagonists as (usually unwilling) queens in their world domination schemes. He is the first KND villain to use this trope; the second is James Nixon McGarfield.