Lotso
Lotso seems like a friendly huggable bear, however, he’s anything but. This character used to love his owner, but ended up lost only to find his owner had gotten another Lotso bear. This bred bad feelings in Lotso, eventually leading to evil and him taking over Sunnyside Daycare and rigging the whole system.
Just like Prospector Pete, this makes him a perfect candidate for the darkness to invade. The downright evil nature of Lotso could make him an incredibly imposing villain for Kingdom Hearts, especially if he gets a Heartless army to command. We don’t know exactly where the Toy Story would will go, but things could get dark if you can fight a Lotso battle in the incinerator he tries to kill the toys with in Toy Story 3. It’d be a stark contrast to the bright and cheery locales of the rest of the world, but could make for a fascinating twist. In terms of Disney villains, you certainly couldn’t go wrong with this bear.