Hey, if Geno lived in the Smash Castle with everyone, who do you think he'd have as roomies? Let's say he can have three. Who would they be? What kind of shenanigans would they get up to?
One idea I love is Mario, Bowser, Peach, and Geno sharing a room. Let's imagine Mallow is in a taunt or something, or maybe a victory screen, too, so he sleeps over often. The gang's all here! Geno and Mario get along swimmingly, rather playful and goofy with one another. They are the bonafide rough-and-tumble good buddies, smacking each other on the shoulders and pitting themselves against each other in stupid contests (arm-wrestling, food-eating, just absolutely silly, dorky things). Geno and Peach also get along very well, often found outside, sipping tea and talking about how eventful the castle is. Geno and Mallow are practically besties, with Mallow looking up to Geno as a brotherly figure and vice versa. Sorry, Frogfucius and King & Queen Nimbus, but Geno has adopted your (grand)son as his little brother now. He would protect that cloud boy with his life.
Then we have Bowser. Ohhhh boy. Geno loves to play little tricks on him, sass him into next week, and push all of his buttons. It's a wonder he's not been turned into a log fire yet. Actually, no, it isn't a wonder, because Bowser does care about him! He's just... uh... not a very affectionate sort towards him, not outwardly. He shows his affection in his own way, such as claiming that, "No one's allowed to make paper out of Twinkles apart from ME!" Geno is well-aware that Bowser has a good enough heart under all those spines and thorns... and that's why he isn't too worried or on edge with him compared to, say, Ganondorf or Ridley. That, and bothering Bowser all the time just tickles him. They're good friends, just... uh, odd friends rofl.
Another option is... old fan requests in one room. K. Rool, Banjo-Kazooie, Ridley, and Geno in one freaking room. I could see Geno finding amusement in K. Rool's flashy antics, even if he largely disagrees with his moral stance. He and Kazooie would have sass-offs, whereas Geno and Banjo would be such chill buddies, hanging out in the forest while Banjo strums at his... well, banjo, before they pass out in a hammock. Ridley and Geno are absolutely at each other's throats, though, and it takes a lot of forced separation to keep them from basically killing each other. (Side note, Geno and Samus get along amazingly well. Geno vents about Ridley's behavior to her on occasion, to which she listens patiently. She feels for him. She knows how that sucker is.)
Gimme your headcanons. Let's go full imagination mode!
ETA: HOW COULD I FORGET ROSIE?! She has to have a room to herself to house a whole bunch of Lumas... and Geno. You walk into the room, and it's just a whole pile of Lumas gathered up with one (1) pair of ribbons peeping out from beneath them all. Geno's a star, too, even if not a Luma. Rosalina may as well take him in as one of her own.