I don't know exactly what match ups Ridley struggles in most, but I think almost any high or top tier character would be a good secondary for anyone. Also depends on how much time you want to dedicate to a secondary character, too. If I had to guess, I think zoners, rushdowns and maybe swords give Ridley a pretty hard time, so maybe picking up a sword character to keep up with the range and/or fast pace of the match with their buttons (but who can also go in vs zoners and wall out rushdown characters well) like Lucina or Roy (depending on whether you're more aggressive or slightly gravitate toward a more defensive playstyle) would work well.