The Interesting Twins From Beneath The Mountain are a pair of Japanese twins, a boy and a girl, who work for Father. They are similar to The Delightful Children From Down The Lane in many ways, including their appearance, naming pattern and manner of speech. Like their counterparts, they talk in monotone, though not always in unison; instead, they often each finish the other's sentences (much like Mr. Wink and Mr. Fibb). It must also be noted that when angered, both twins can raise their voices to the point of loudly yelling should the situation call for it.
They both wear matching blue, Japanese school uniforms of the sort based on European military uniforms (the sister wearing a skirt and her brother wearing shorts) along with large black shoes. They also have birth-marks on their faces which parallel each other. The right side of the girl's face is dark and goes in a curve under her nose, then curves downward around her mouth (muzzle) with her nose (which is in the darker area of her face) being pale like the left side of her face. The boy has basically the same, but the colours reversed, like Yin-Yang symbols. The girl wears her hair in a pair of pigtails, and the boy has a pudding-basin haircut. While it is unknown if they are good fighters, they have been shown to be able to fit into extremely tight spaces that seem somewhat impossible at times. They are also shown to be very acrobatic, one example being in Operation: T.R.I.P. when they are disguised in an overcoat and hat, with one standing on the other's feet while the bottom twin "walks" using their hands, whilst following Kuki.
While largely lacking in distinctive personalities, the twins act as two separate beings, unlike the Delightful Children. They are most likely Delightfulized and, if so, are possibly the only "normal" delightfulized children seen in the series. Instead of ever saying each other's names, they simply address each other as "Brother" and "Sister", respectively.
The sister is noticeably more volatile and easily angered than her brother, and is typically the more dominant and prideful of the two. When in multi-part disguises, she is often the one in the more prominent and/or extensive one (such as when they are disguised as a woman and small dog, she is the woman while her brother is the dog). If stressed to a certain point, the sister will begin to show signs of paranoia, such as searching a trash can that Numbuh 3 threw their lollipop tracker into, while thinking Kuki was really inside it despite the lack of reason.
The brother is seen as the calmer and more sensible of the two, but also more timid, rarely speaking unless prompted by his sister or other circumstance. Though, while he tends to act as the voice of reason between the twins, his sister doesn't listen to him when she is extremely angry. He usually acts as the more minor part of their disguises. He also seems to have a yin-yang watch which summons all of their ships.
When the twins fail at their first seen mission in Operation: T.R.I.P.. The sister completely breaks down, crying and calling herself a failure, then throwing a tantrum. Her brother then proceeds to comfort her, and displays a rare moment of defensive anger at the Kids Next Door for causing his sister's distress, calling them "monsters" for not allowing his sister to succeed (showing a measure of selfishness towards himself and his sibling's feelings, as they intended to help Father destroy them), and seeming to take his sister's role as the more dominant one, vowing revenge.
They both seem to share a measure of pride and over-confidence in terms of their abilities, calling the bizarre yet debilitating events that hinder them "minor setbacks" and assuring Father that they are "twice as good as required" and that "failure is an impossibility". It takes the reveal that they were literally unable to achieve their goal from the beginning to get them to relent, following Kuki through the entire episode despite the painful and humiliating incidents they go through, showing fierce determination.
The Interesting Twins are masters of disguise and espionage. They possess many mechanical suits that perfectly replicate human appearance and behavior and show no discernible differences from actual humans. When these are not available, they have been seen fully capable of utilising common appliances, such as mops, rugs, and drapery.
In Operation: T.R.I.P., they are tasked 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, they discover that she is apparently just 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 presumably beat them up.
At the end of Operation: R.E.C.R.U.I.T., "Bobby", supposedly a boy wishing to join the KND, is revealed to be the Interesting Twins in disguise. After going through the tedious test given to them by Sector V, they give up on their mission out of frustration, but swear to return once more.
In Operation: D.I.S.G.U.I.S.E., they disguise themselves as Numbuh 1 and infiltrate Sector V's Treehouse in an attempt to sabotage the KND, but after several other Numbuh 1's appear claiming to be the Interesting Twins, they give in, once again jumping out the window and falling a long distance. It is revealed at the end that the other Numbuh 1's were Sector V in disguise, who had fooled the Twins once again.