Fox is pretty well rounded in this game and doesnt really have any true counters, but imo he loses slightly to Diddy, Sheik, Mario, Luigi, Mac, Pikachu, Toon Link, and maybe one or two more. I think he might also lose to Dedede but Im not sure on that one. Hardest match up is prob Sheik just because she controls neutral better, has great air maneuverability with down b, and needles are amazing for punishing.