This article focuses on the relationships between Numbuh 3 and other members of the show.
Mr. Sanban loves Kuki very much, but is very crabby and easily set-off, and it is usually his wife or Kuki that calms him down. He regrets the very day that Kuki got her first Rainbow Monkey and hates them overall due to his daughters' constant "bickering" over them.
Genki (or Mrs.) Sanban is the mother of Numbuh 3 and loves her daughters very much. She is the one who buys all of her daughters' Rainbow Monkeys, feeling a little jealous about never having any dolls to play with when she was a child as seen in Operation: C.L.U.E.S..
Numbuh 3 shares a very close bond with Mushi, her younger sister, and treats Kuki as her "best friend", both having similar personalities. Mushi and Kuki do fight at times though, and Mushi might not feel as close as Kuki does.
In Operation: S.P.A.C.E., when Mushi tells Cree how sisters are supposed to love each other and be "bestest" friends, but after Cree is shot out of the trash shoot by Numbuh 5, Mushi reveals she made that all up. Mushi's true feelings toward her sister were revealed in Operation: C.L.U.E.S., when Numbuh 2 deduced that Mushi was the one who "murdered" her sister's Posh Party Rainbow Monkey , out of jealousy and envy.
Throughout the series, it was obvious that Numbuh 3 has a crush on Numbuh 4, but both were respectively too shy and stubborn to admit her feelings. Kuki and Wally have extremely different interests and personalities, with Kuki loving Rainbow Monkeys and being a girly-girl while Wally enjoys all things gross, violent, and generally hates Rainbow Monkeys. Kuki tends to hug Wally when something flatters her, making Wally blush and shy. It's also worth noting that among Sector V, Wally and Kuki are the only members that regularly speak to each other using their real names rather than their code names.
In Operation L.O.V.E., when Numbuh 4 interrupted the school play that Kuki was co-starring in, he tried to confess that he loved her, but asked, "What are my parents doing here?". After the play, he offered to sing the last song in the play with Kuki, which she couldn't originally sing due to his interruption. Also, in Operation: I.N.T.E.R.V.I.E.W.S., it is revealed that when they grow up, she marries Numbuh 4.
There are a number of other occasions when Numbuh 4 appears to attempt to confess his feelings for Numbuh 3: Operation U.N.D.E.R.C.O.V.E.R. and Operation H.O.S.P.I.T.A.L.
Kuki might have had a crush on Ace, though it is unknown for sure. In CAKED FOUR, she was seen flirting with him. And she also hung out with him when Ace was pushing her on the swing. There isn’t a lot known about if Kiki actually had a crush on him.
In general, Numbuh 1 seems to view Numbuh 3 as a very close friend. It could even be said he treats her like a little sister. Apparently, they both met at a young age. He occasionally, but notably, refers to her as "Kuki" more than the other members. It was hinted by Numbuh 5 at the end of Operation: W.H.I.T.E.H.O.U.S.E. that Numbuh 1 may have/had a bit of a crush on Numbuh 3. This came about when Numbuh 5 was asking Numbuh 1 questions to prove that he actually was Numbuh 1, and she asked;
"Who had a crush on Numbuh 3? And dont say Numbuh 4....."
Nigel responded to this by narrowing his eyes, possibly suggesting that it was him. This is unconfirmed, and it is not known if Numbuh 3 ever returned his feelings if he had them.
He and Numbuh 3 share a similar relationship that Numbuh 3 shares with Numbuhs 1 and 5. Like Numbuh 5, he knows of her and Numbuh 4's crush on each other, and also openly teases Numbuh 4 about it. Both he and Numbuh 3 foster parented Numbuh 6 (Bradley) in Operation: C.A.M.P., and behaved like a typical bickering married couple for much of the episode.
Being the only other girl in Sector V, Numbuh 5 is good friends with Numbuh 3, despite being opposites from each other. Unlike Numbuh 3, Numbuh 5 isn't as girly or ditzy, and easily gets annoyed by Numbuh 3's antics. Nevertheless, Numbuh 5 cares about Numbuh 3 like a sister and tries to comfort her, as seen in Operation: H.O.M.E. when her First Rainbow Monkey was broken. Numbuh 5 is aware of Numbuh 3 and Numbuh 4's crushes on each other and would often teases Numbuh 4 about it.
There is almost no evidence that Numbuh 3 is friends with Numbuh 362, other than seldom talking in a few episodes and having a typical superior-subordinate respect for each other. Numbuh 362 did appear at her house along with several operatives in operation H.O.T.S.T.U.F.F., when Numbuh 3 turned up the thermostat (turning her house into a lava-filled inferno). In the end of the episode, Numbuh 362 congratulated Sector V after saving her.
Not much is known about Numbuh 3's friendship with Numbuh 23, other than that she also attended Numbuh 86's sleepover and have an interest of Rainbow Monkeys. Numbuh 23 seems to know about Numbuh 3's crush on Numbuh 4 and giggles when Numbuh 3 was asked by Numbuh 12 whether or not she like-liked Numbuh 4.
When Numbuh 86 first appeared in Operation: F.U.G.I.T.I.V.E., she worked alongside Numbuh 3 and the others when they tried to capture Numbuh 206, and appeared to have a mutual respect for each other until the end when Numbuh 86 was being unfair to the boys of Sector V. Kuki was invited to Numbuh 86's sleepover in Operation: S.L.U.M.B.E.R., which developed the relationship between the two girls. In the episode, Numbuh 12 betrayed the KND, who was considered a friend by Numbuh 86. Afterward, Numbuh 3 tries to cheer her up and agreed to be her friend when no one else would. They are good friends since then, and also shown to share the same interests, such as Rainbow Monkeys.
Not much is known about the conflict between Chad and Kuki, but when Chad was Supreme Leader, Kuki along with multiple KND operatives admired him. However, when he betrayed the KND, Kuki thought of him as "a big meanie". in Operation: S.L.U.M.B.E.R., Numbuh 3 made fun of his bra (Battle Ready Armor). Chad was one of the few supreme leaders (alongside Numbuh 362 and Numbuh 100) who welcomed Kuki into the KND.
Chester may be considered Numbuh 3's biggest enemy since he was the one who threatened Numbuh 6 and almost killed her when he tried to feed her to sharks as a giant hamburger. Therefore, Numbuh 3 greatly hates him.
In Operation: B.E.A.C.H., King Sandy kidnaps Numbuh 3 to make her his queen. Until she learns his intentions are real, she believes he is simply playing, and is even flattered he would choose her for a bride and future queen. When he returns in Operation: K.A.S.T.L.E., he again kidnaps her and attempts to marry her like before, though at this point she is fed up with him and openly despises the proposal (though when he confesses his love for her in the dungeon, she admits his confession was "so sweet"). She seems to hate him even more when he falls in love with Mushi, not so much because he changed his mind, but because he called her old and ugly.
Though appearing to battle against them, Kuki hasn't shown any visible malice or direct relationship to the DCFDTL, except in Operation: L.O.V.E.. It is notable that one of the children, mainly the blonde girl, share similar body types and hair styles as Kuki, though Kuki's hair is black.