If you don't like any of the secondaries, you can just solo main fox honestly. But having a secondary is great, pretty much everyone at a top level has one. If you think falcon is fun, don't drop him if he isn't a "good secondary". I play charizard still even though he probably doesn't do much for me, but I love playing zard. But for matchup purposes, you want a character that does well against rosa, luigi, Kirby, and ness, because these tend to be the MU's that fox mains say are hard. Notice similarities in these characters? They are all floaty, hard to combo, and relatively light. Therefore, I think you should use someone who doesn't rely on combos, or has combos that work on floaties, while also having the kill power to take advantage of their light weights. There are a multitude of characters that fit this role, but personally I use link. Link has kill power everywhere, and his zoning capabilities are especially good against luigi, Kirby, and ness, characters that can struggle to approach. He also doesn't need his combos to win, and he is heavy enough to take a beating and keep fighting. It's really up to you which character you pick, but in my opinion, you should pick someone with these traits.