To be fair, the bear and sword boys are some of gaming's oldest heroes and the thief is from one of the best JRPGs ever as well as one of the best games of the 21st century.
I can't really excuse the plant, but give it some credit, this is the first time it's been playable in any game.

Yeah, he was actually very apologetic about it as he really seemed to want to put Rex in but it was too late to add him. Can only hope he'll make it into the next Smash whenever it happens.