Interesting Twins From Beneath the Mountain

The Interesting Twins From Beneath The Mountain are Japanese counterparts of the Delightful Children From Down The Lane who appeared in two episodes of Codename: Kids Next Door. They are a brother and sister who are expert spies and incredible masters of disguise, being able to fit themselves into perfect mechanical replicas of anything, and usually finish each others' sentences. They appear to be permanently "delightfulized" by Father, but not to the extent of the Delightful Children (who were a one–time result of the first delightfulization chamber made having much stronger effects than intended and destroying itself in the process of creating them). Although they constantly do their best to stay cool, calm and collected, the twins have implausibly bad luck and seem to end up quitting all their missions out of frustration.

In Operation: T.R.I.P., they were tasked by Father with stalking Numbuh 3 while she visits the elusive Japanese Kids Next Door, and upon discovering their location, contacting an army of ninja mercenaries that Father has hired to attack them. However, they are continually plagued by bad luck and improbable setbacks throughout their mission, and when they finally follow their target to her destination, only to discover that she is visiting her grandmother.

They begin to complain to her about this before the "grandma" reveals herself to be the members of Sector V in disguise. The girl then breaks down and cries, and the boy calls out the Kids Next Door for causing this before they both leap out of the house's window, vowing to return. However, as the house was on top of a hill, the twins fall a long distance before landing on their backs, at which point the boy laments that their situation cannot get any worse. They are then approached by the various characters seen on the trip who inadvertently caused trouble for the twins, who reveal themselves to be the Japanese Kids Next Door in disguise and proceed to beat them.

The Interesting Twins made another appearance in Operation: R.E.C.R.U.I.T., where they disguised themselves as a KND–obsessed boy named "Bobby" who desperately wanted to join the group in an attempt to infiltrate them, but the Kids Next Door were on to them and put "Bobby" through a ridiculous and painful "initiation test" constituting a complex parody of The Matrix, which eventually made the twins angrily quit out of frustration once again.