Ok so there's a couple things I'd like to clarify.
First off, why this style of game? Honestly, I'd much rather go with a 2D character action game inspired by Devil May Cry, Bayonetta, or Metal Gear Rising: Revengeance. The problem with that is the complexity of those games. Even if we were to simplify it, it's still a bit too much for a first game. So we decided to instead go the route of Kirby with it for now.
As for why the characters are nude, I understand the thought process behind everything needing a valid reason via lore or setting, but I don't think everything needs some kind of deep reason. I personally feel as if they can be nude just because they can. Plus I want to express that aspect of my own life, and thankfully my friends are fine with that.
The world is heavily influenced by the likes of Dragon Quest, One Piece, Sonic, Mario, and Kirby, so it's very colorful and warm feeling with a lot of cartoonish wackiness to it. Many of the human characters and humanoid characters are at least roughly partially nude (shirtless, bottomless, robe, etc) and nudity isn't uncommon. There's many designs in which it's there but isn't the forefront, or isn't even entirely shown as much as either implied or conveniently censored without that being the goal (such as the elderly shop keep woman sitting behind the counter so it's only about shoulders up).
Although I can totally understand the concern for making it feel natural and there needing to be a reason to justify it.
Initially, the concept was still a character action game and it was starring a duo that you could pick between. One was a magic wielding woman and the other was a scythe wielding man. They together represented the concepts of opposites by having elements of life and death, love and hate, mind and body, plant and animal, fire and ice, etc. They were the only ones nude because they were demigods among a dying land where the surviving remnants of humanity held on amidst the world being flooded by monsters. However, that felt a little weird with them being the only nude characters due to status but everyone else being pretty fully dressed. So I changed it.
The second iteration was where they're human-like androids with outfits inspired by Birdie in Birdie the Mighty: Decode where it was pieces of material covering different parts of the body. They both control the same and are aesthetically different aside from a few animation changes and are now fighting against a bunch of advanced androids like them who have been corrupted by a virus to kill off the planet and put it out of its misery, these other androids also being able to transform like a mix of Transformers and those battleship anime girls from stuff like Azure Lane. However, I feared this would be too close to Nier: Automata (despite me having only beaten the first story in it).
So, I went to friends for help and the third iteration became a story with a man accompanied by the soul of a woman who fight alongside each other in a world that looks like a strange and twisted version of modern earth but no humans in sight, instead populated by anthropomorphized tadpoles and monsters. All he had was a pair of shoes and all she had was a bow in her hair, so they both had at least a little flair. He would fight with physical attacks and she would use spiritual energy alongside his attacks. The plot was something about getting to the giant tree at the center of the country and finding their missing body and heart in order to become their whole selves again. The reason for the nudity in this one being that they've been brought to this world from another. Which was also a bit of a problem, as I felt it was a little generic.
But then that one kind of fell off a bit and became difficult for me to handle alone, so I asked friends for more direct assistance. The next iteration was just a guy exploring a fantasy world with a magic sword that can also cast spells of various kinds, simplified controls to be an action platformer, and giving him some gloves and boots. But I found it to be a little uninspired since it didn't have any basis for what we'd want it to play like, so we all looked at good games and how they kind of function and what we could pull from that. That's how it got to the current idea of Kirby x Mario.
I still most favor the third iteration to be honest, but I really like the feel of the world with this fifth iteration more than any prior. It's definitely been testing me, fighting off habits of mine that would impede the project more than help it I think. It's also the first one with more than just the protagonist(s) and maybe another character having nudity, making it more of a regular thing in the world like people being sleeveless.
Sorry this is long winded and going all over the place, I got distracted several times over while typing this and lost my original point many times.