It's the &Knuckles mod! Now with 200% more Knuckles!
This mod, simply put, adds an AI Knuckles partner that follows every character all the time. Except in Tails' first four levels, then you're racing Knuckles instead. And he's absent from Twinkle Circuit and Sky Chase.
This mod may make you chuckle, but don't go into cutscenes with it, it'll crash.
My original plan was actually to have a whole group of Knuckleses following you, but that would require custom AI objects.