When Finn traveled to Gene's mansion, he assumed Finn was just another greedy human. However, Finn came to ask Gene to free Jake, who had been imprisoned in an indestructible cage, Gene was touched. He gets along well with Finn and enjoys having fun with the human. It is quite obvious that Gene has a lot of respect for Finn and has gone out of the way to protect him on a few occasions.
Out of boredom, Gene will occasionally create a new fanciful game such as "Gravity-Less Hockey" or "Space Ball" in order to pass the time with Finn and Jake.
After Gene's house is blown up "By a Goblintrollninjaghostthing", he takes up residence in the Tree Fort's laundry room, presumably due to his enjoyment of Finn's company.
Like Finn, Gene gets along well with Jake, and seems to genuinely enjoy his antics. Gene has claimed that Jake is important to Finn's mental well-being and that the two make a great team. Jake, meanwhile, thinks Gene is pretty cool and actively participates in his games.
When Gene reversed the directions for everyone in the Candy Kingdom, Princess Bubblegum was enraged with him and demanded that he fix it. Although he does eventually revert the kingdom to how it was, Gene still refers to PB as the "anti-fun" and does not get along with her well. She often claims (correctly) that some of his antics are dangerous, and has feared for Finn's safety when he is near Gene.
After Gene moved in with Finn and Jake, Princess Bubblegum's anxiousness reached a boiling point, so Finn suggested the two hang out for a day. After Gene showed knowledge for science and revealed he's just trying to have fun, Bubblegum lightened up to him a slight bit, but is still cautious about his antics.
Elf Prince & BMOEdit
Gene seems to be good friends with both Elf Prince and BMO. He was allowed to come to the wedding of Elf Prince, prior to their first meeting on screen together, which was not seen until the next season. Gene thinks of BMO more as a funny playmate, and thinks he is cute. Both Elf Prince and BMO are game buddies of him.
Hudson Abadeer, Peppermint Buttler & DeathEdit
Gene, alongside Mr. Abadeer, Peppermint Buttler, and Death, were seen together in a picture on Gene's mantle. Their exact relationship remains a mystery, but it can be presumed they are all on good terms. Gene knows how to access the Land of the Dead and the Nightosphere, indicating a greater knowledge of both worlds.
The relationship between Marceline and Gene is very complicated. It was revealed in "Only Three Wishes" that Gene and Marceline had met each other in their teen years, when Marceline was residing in the Nightosphere with her dad and had not yet moved out. Marceline owned Gene because she found his lava lamp but knew he would be gone if her third wish was granted, but wanted to wish for his freedom.
They grew very close to each other, but never got into a confirmed relationship. When Marceline wished for Gene's freedom the Grand Master Wizard appeared and took Gene away to stand trial. Marceline and Gene met again later but didn't want to continue the relationship, as it would be too awkward. They however became 'friends with benefits'. Till the day of today they often hang out, but don't talk about their relationship to anyone. She doesn't love him, as she loves Princess Bubblegum, but different, as they still have their 'little adventures'. Which is something only PB is aware of.
As a being of considerable age and antics, Gene has several enemies. He has stated that the Ice King, while a bumbling fool, could still be dangerous in the future. He feels sympathy for Ice King and sees he is just a misunderstood man. However when he was trapped by Ice King he got bored real quick of his talking, as he says that his rambling would "even make a dead guy commit suicide."
He feels really sorry for Ice King because he just feels that he has been through a lot. He doesn't consider Ice King as a treat but still someone to be aware of.
Grand Master WizardEdit
The Grand Master Wizard is the disciple of magic, and all magic goes through him. He thought Gene the basic aspects of being a genie. However when he broke the rule of falling in love with one of his 'clients', Gene immediately was sent to stand trial. He doesn't hate the Grand Master Wizard, but also doesn't like being with him in one room.
Gene fought Magic Man at some point in the past although duo to unknown reasons. Their battle ended in a tie, and it always has been since. They often try to get in each others ways, although Magic Man a lot more than Gene nowadays. When he fought Finn and Jake, Gene managed to save them and threatened to turn him into a piece of taffy should he ever mess with Finn again.
They had several encounters afterwards. Most of Magic Man's plans involve getting into the Taffy Tower and steal one of the artifacts there. What he is going to use it for is unknown, but every time he luckily gets stopped by Gene and sometimes Finn and Jake. Their rivalry may also be frenemy-like as they also taunt each other very much. But they never acted friendly in any way to each other.
The worst nightmare of Gen is "something that doesn't exist" as referred by him. This thing seems to be the Goblintrollninjaghostthing. It is unknown why Gene has such a big fear for the creature, but the creature seems to be constantly stalking him, which is driving Gene sometimes mad.
When he was late to meet Finn and Jake, Gene claimed that he was busy battling Professor Pirate again, and later mentions the Gopher Lord as another old foe.