After Super Smash Bros. Ultimate creator Masahiro Sakurai announced Kingdom Hearts' Sora as the game's final DLC character, he took to Twitter to post images of Kirby with Sora's abilities copied — as is tradition whenever a new fighter releases. However, while these images show Kirby making use of Sora's wide range of magical abilities, they cleverly don't show the pink puffball using Sora's Keyblade, which confused those who've never played Kingdom Hearts.
In a recent tweet with images showing Sora and Sora-Kirby practicing magic together, Sakurai clarified why Kirby doesn't use Sora's Keyblade whenever he uses his abilities. The simple answer is that he can't. While Kirby's copy abilities are stronger than their original owners, he can't force Sora's Keyblade, or any others for that matter, to come to him, and anyone that's played Kingdom Hearts likely already knows the reason why.
"Only a chosen Keyblade wielder can wield a Keyblade" is more or less the translation of Sakurai's tweet, and it falls in line with Kingdom Hearts lore. Keyblades have minds of their own and will refuse to work for anyone other than their chosen wielder or wielders with close ties to their chosen. In other words, before Kirby can run around with his new friend's Keyblade, he'd have to be capable of wielding one without a copy ability, and to do that, he would have to have a "strong heart."
Sakurai confirmed my theory from last week: "Only a Keyblade Wielder can use a Keyblade" from SmashBrosUltimate
Kingdom Hearts has tackled this "copy" concept in its own canon as well. Kingdom Hearts 3 Re:Mind's final boss, Yozora, can temporarily take Sora's Keyblade away from him and copy its magic-producing abilities. However, because he isn't a chosen Keyblade wielder, Yozora can't physically use the Keyblade or keep it away from Sora for very long as the weapon will always return to its chosen wielder even if the wielder themselves doesn't want it.
It's important to note that while Kirby has a "pure heart," that is not synonymous with a "strong heart," as individuals like Master Xehanort can freely wield hundreds of Keyblades while being as evil as can be. Now, if it were only up to Square Enix, Kingdom Hearts' director, Tetsuya Nomura, might have designed a Keyblade for Kirby to use, but as it stands, Disney might make such a thing complicated as they have a controlling interest over the Kingdom Hearts property. Some speculate the company was why it was so hard to get Sora into Super Smash Bros. Ultimate in the first place.
Super Smash Bros. Ultimate is available on Nintendo Switch.