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Basketball[]
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In "What's Up Bell's String?", Basketball witnesses One teleport Bell and Bomby to another place, after which she attempts to convince Tennis Ball and TV that she saw One. Later, after seeing One take Fanny away, she attempts to explain what she saw to Tree. Basketball and One would not directly interact until the episode stinger, where One notices Basketball spying on her, and offers Basketball a chat, before teleporting her away.
In "Category One", One is blackmailing Basketball into making a deal with them or else they make Robot Flower even more destructive as well as making Basketball missing just like her other victims.
Bell[]
In "What's Up Bell's String?", Bell asks who One is, with her responding by saying the two should get to know each other.
In "Category One", it can be presumed that Bell, alongside Bomby, are stuck in One's Field, meaning that Bell could be resentful against One.
Bomby[]
In "What's Up Bell's String?", One suggests that she and Bomby get to know each other, during the interaction Bomby is noticeably fearful of One's presence.
In "Category One", it can be presumed that Bomby also could be stuck in One's Field, meaning that Bomby, just like Bell, could be resentful against One.
Barf Bag[]
Even though they haven't interacted yet, it could be presumed that Barf Bag could be resentful against One, since she trapped her and Needle in the same area with Bomby and Bell.
Donut[]
In "Bottle For Dream Island", One tries to make a deal with Donut. She mentions that she had held off on kidnapping him because she suspected he was the type to cause problems. Indeed, Donut repeatedly denies the deal because One doesn't elaborate on the details of the contract, as well as what she gets if he were to accept it. Donut tries to leave, but One forces him to stay in the Dimension. One tries to manipulate Donut into thinking about himself and accepting the deal, but he retaliates by kicking her in the face. Enraged, One rips his legs off, and kicks him back to the A-twos-ment Park. Later, One can be seen angrily crossing Donut's name off her list of targets, telling herself he was just a last deal for luck anyway.
Fanny[]
In "What's Up Bell's String?", One takes Fanny with them to a more "casual" place. In there, One offers a new mouth to her in exchange for a favor we have yet to see. After regaining her mouth, Fanny says she hates the deal and asks One to go away angrily.
In "The Power Of Four", Fanny and Ice Cube are seen telling Two about One.
In "Bottle For Dream Island", Fanny shows her investigation about One to Tennis Ball. However, her notes are all unreadable on his end, showing up as random scribbles in his eyes. In this scribbled mess, there are some hidden messages that (once solved through a Caeser Cipher) appear as threats toward Fanny. With one of them even calling Fanny a snitch, it's clear that One is upset that Fanny is trying to alert the other contestants of her presence. After Tennis Ball leaves the room, the drawing of One on the whiteboard winks at Fanny before teleporting away, with Fanny stating that she "hates her".
Four[]
In X Finds Out His Value, One and Three were jealous and angry after Four called X value 7 lucky, because they are equal to 13 (Known to most as an unlucky number).
In "Thanks for 1,000,000 subscribers!", Four asks 'Now, where's One?' with a rather happy tone, possibly meaning that they didn't have a bad relationship. (At least on Four's side)
In TPOT, it is unknown whether One still holds a grudge against Four, but probably yes, because in "Category One" when she counts "4" she becomes irritated.
In "I SAID CAREFUL!!!", a sticky note of One is seen on Four and X's fridge without any scribbles unlike Three, again possibly hinting that they don't necessarily have a bad relationship from Four and X's side. (However, the sticky note could've been added by One themselves to spy on Gaty)
In "Bottle For Dream Island", Four pulls up the moon sticker on the fridge, and realizes that One is missing. He then has a look of skepticism as a result of this.
Gaty[]
In "I SAID CAREFUL!!!", One is seen stalking Gaty through a photo, possibly planning to steal her away.
In "Seasonal Shift", One kidnaps Gaty into making a deal with her. Gaty refused to take the deal as it may be dangerous, which annoys and upsets One. One keeps trying to convince Gaty into signing a deal with her until she blackmails her into signing the deal. Gaty later signed the deal for the sake of reverting everything to the way it was. Gaty asked One if she could send her back into the hotel, but One doesn't respond. She presumably sending her into One's Field alongside the other eliminated contestants taken by her.
Leafy[]
In "Category One", One is implied to have taken away Leafy in the storm.
Ice Cube[]
In "What's Up Bell's String?", One offers Ice Cube new legs in exchange for accepting One's deal. Ice Cube does not show much emotion about this and willingly accepts.
In "The Power Of Four", Fanny and Ice Cube are seen telling Two about One.
Needle[]
Even though they haven't interacted yet, it could be presumed that Needle also could be resentful against One, since she trapped her and Barf Bag in the same area with Bomby and Bell.
Seven[]
In XFOHV, One seemed to have been angry that Seven was lucky but them together with Three was unlucky.
Non-canon: In "Thanks for 1,000,000 subscribers!", Seven said that One is their best friend. This might be a joke, but we don't know much.
Six[]
In "Bottle For Dream Island", when Needle tells Six about One sending them here, Six looks at the door, looks terrified and tells them they have to go. While running to Four's door, Six says that One was meant to be gone, which implies that he and the other algebralians have a negative relationship with One, and that One was possibly imprisoned in the Moon intentionally, likely due to her desire for power.
Three[]
One and Three
One and Three were seen together in X Finds Out His Value, they muttered "Hmp!" showing they were unhappy because they are equal to 13, which is considered unlucky, as opposed to X's value of 7, which is a lucky number. None of this shows they are friends; this is simply a moment of agreement.
Two[]
In "What's Up Bell's String?", while One is talking to Fanny, they say that they're an old friend of Two. Also, if you go to the official merch website, you can see that the description of the plush also says that they are old friends. However it is possible this is simply a lie to get Fanny to trust her.
In "Category One", One is saying her power is "not enough" when the scene shows Two, implying they're jealous of Two's power.
In "Seasonal Shift" She kidnaps Gaty, Two's closest friend. Two currently has no idea about her possible whereabouts, its likely this will put both Two and One at odds with each other in the future.
X[]
In XFOHV, One and Three were looking at Four angrily after Four called X value 7 lucky, because they were equal to 13 (an unlucky number).
In TPOT, it is unknown whether One still holds a grudge against X.