That mod is Sonic Versus Heartless by @somnusNemoris_. Using Kingdom Hearts 3 as a framework, it places Sonic in the oversized shoes of the series' regular protagonist Sora. Not only that ...

All the other characters are their normal Sonic-selves, while Shadow is almost comically miserable. “This ugly heartless creature – I may look like a hedgehog but I’m really no different ...

Maybe that makes me heartless, but not as heartless as Chris Hernandez, the Sonic lore manager who reaffirmed that Sonic is, indeed, without a home, after an incorrect draft of a comic made it to ...

The Kingdom Hearts Tamagotchi debuted in Japan in October 2022, and the means of getting the Heartless form for Sora appeared. However, it turns out it “ends” the game for people when they do.