These are the relationships Banana Joe has with other characters in the series.
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.
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. In "The Oracle," Joe mentions how Barbara had made all her paintings, despite not knowing she was painting the future.
Although Banana Joe was initally unfriendly towards Gumball, 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. Regardless, Gumball does not seem to think of Joe too highly and tends to neglect him, as in "The Promise" or "The Ex," where he was willing to harm Joe for the sake of befriending Rob.
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."
Tobias looks like a frenemy of Banana Joe, because sometimes they are seen together, and other times Tobias mocks him. 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.
In "The Banana," he along with Gumball and Darwin go to see Mr. Small in his office. While their relationship is mostly unknown, they have interacted on some occasions.