These are the relationships Banana Joe has with other characters in the series.
Family
Banana Barbara
Banana Joe appreciating his mother
Banana Barbara is Banana Joe's mother. Joe is particularly caring towards his mother, however oblivious of her physical condition and tendency to substain injury, such as in "The Mothers," where he tried to get her to be declared the greatest mother, and still thinks that she is the best mother, despite her losing the "Mom-Off." In "The Oracle," Joe mentions how Barbara had made all her paintings, despite not knowing she was painting the future. In "The Future," they are shown to be extremely close, as Joe falls into despair when his mother is kidnapped. With Gumball and Darwin's help, he seeks her out, and Barbara tries to help by painting clues into her magic paintings. When Joe finds her, he is enraged at her kidnapper, Rob, and attempts to fight him himself, but he is flung away due to being too weak to fight. When he finally has Rob cornered thanks to Gumball, he erases him from reality with no remorse, showing just how much Joe loves his mother.
Banana Bob
Banana Joe and his father
Banana Bob is Banana Joe's father. Like his son, he is not the brightest character (such as in "The Bus," where he accidentally gets run over due to not knowing how to handle a pair of binoculars), but he still tries to act as a caring father. He sometimes annoys Joe by attempting to join in his attempts to impress people online, but the two still appear close.
Friends
Tobias
Banana Joe angry with Tobias
Tobias has always been Banana Joes closest friend. In "The Pressure," they were the two Pals Before Gals members who kept up with their pact. Later in "The Club," they are both seen playing around in a friendly way in the football club. In "The Promise," however, it is mentioned that Tobias insulted Banana Joe's mom, and in "The Mothers," the two compete over who has the better mother, showing rivalry between them. These are only minor instances though and they have prior gotten up to much tomfoolery together.
Darwin
Banana Joe and Darwin eating ice cream
For the most part, Darwin and Banana Joe seem to share a very good friendship, though like his relationship with Gumball, he was initially a bully towards them (as in "The Gi," where he relentlessly made fun of the pair's costumes).
In "The Banana," Gumball accuses Banana Joe of chewing Darwin's pen, but Darwin didn't seem to mind. Later, though, when he sees Joe trashed his desk and homework, he goes through a rampage through Joe's stuff in hatred and revenge. Later, still, when Joe offers him a new pen, Darwin feels remorse for what he has done. When Joe reaches his locker, he attacks Gumball and Darwin, but accidentally knocks himself out. Finally, Gumball and Darwin beat themselves up in an effort to make up with Joe, which succeeds, and they become good friends again at the end of the episode.
In "The Promise," it is revealed that Darwin cares a lot about his relationship with Banana Joe - when Joe gets mad at him, he does everything in his power to restore their friendship, to the point of breaking his promise with Gumball to play The Tale of Zelmore. Regardless, he still abandons Joe for the sake of retaining his affection for Gumball. Both "The Banana" and "The Promise" seem to reflect that Darwin is not above prioritizing over Joe for his or others' sake.
From that point onwards, Darwin and Banana Joe seem to be pretty close friends, often hanging out and going for ice creams, as seen in "The Roots." Joe also plays harmless, friendly pranks on Darwin, such as the one he pulled in "The Butterfly." Despite this, Darwin is almost completely unbothered when Joe is run over by an ambulance in "The Advice," literally shrugging it off due to it helping them to not hurt Mr. Small's feelings.
In "The Future," he and Gumball help Joe look for his mother, getting into character as Barbara.
Gumball
Gumball laughing with Banana Joe
Banana Joe was initally unfriendly towards Gumball, such as in "The Gi," where he mocks him (and Darwin) for wearing karate uniform to school, or in "The Photo," where he attempts to ruin Gumball's school photo as revenge for ruining his own. However, as the series progressed, the two soon began to befriend one other, and as a result, Gumball is now one of Banana Joe's closer friends. On the surface, Gumball does not seem to think of Joe too highly and tends to neglect him, as in "The Promise," where he demands Darwin spend time with him over Joe (although he does seem concerned at first when Joe is angry at them) or "The Ex," where he was willing to harm Joe for the sake of befriending Rob, or in "The Vegging," where orders Joe to point his phone at the TV so that he can watch it after the Wattersons' TV is destroyed by lightning, even asking Joe to stop breathing so he can hear it better. Joe does all this without question.
Despite this, "The One" shows that Gumball actually does care about Joe, as he sees him as his fourth closest friend, after Sarah, Carrie, and Darwin. In "The Future," he and Darwin help Joe look for his mother, getting into character as Barbara.
Idaho
Banana Joe and Idaho in "Halloween"
Banana Joe and Idaho are very good friends, as they are seen hanging out many times in the series, such as dancing in "The Party," asking for candy in "Halloween," talking in "The Society," and also sitting together in the cafeteria in "The Weirdo."
Carrie
Banana Joe and Carrie sitting together
Carrie seems to be friends with Banana Joe, as in "The Return" they are sitting together, implying a positive relationship, but not much else is known besides this.
Enemies
Rob
Banana Joe and Rob were never seen interacting until "The Ex," when Rob chooses Joe as his new nemesis after reconciling with Gumball in "The Rerun." Rob repeatedly attempts to harm Joe but is thwarted both by Gumball (intentionally) and Joe himself (unintentionally). Joe's eccentric behavior (such as removing all of the sesame seeds from his burger before eating it) frustrates Rob greatly, although Joe does not notice anything until Rob strangles Gumball outside of his house, which shocks him. However, Joe begins to requite Rob's dislike (even though Rob seems to no longer particularly care about him) when Rob kidnaps his mother Barbara in "The Future." As soon as he discovers that Rob was responsible, he becomes aggressive and confrontational, although Rob quickly neutralises him with a crowbar. Once Rob is defeated, Joe is quick to erase him from existence using his mother's powers, though he seems quite upset after he realises what he has done.
Other
Mr. Small
In "The Banana," he along with Gumball and Darwin go to see Mr. Small in his office. Small can also be seen comforting Joe when his face is removed. While their relationship is mostly unknown, they have interacted on some occasions.