Roy is especially strongest against Fox and Falco, cause his combos can be fully used since they are so heavy.
Easy little combos to use to get their damage up, is to Chain Throw 2 or 3 times into his forward smash. He can use his down tilt to pop them up and forward smash them. If he can wave dash, he can down tilt into a wave dashed grab and start a chain throwing combo.
Also against a Fox, if he does his double edge dance, in the motion of Forward B, Up B, Forward B, its a gurantee'd hit on the last B and with decent knock back.
Also, early percents, he can combo with his Smash up A attack and get it in 2-3 times (Usually only 2 because they know to DI by the third), And its always a nice show stopper.
He also has a variety of finishes for Fox.
First off, Fox is one of the only people able to combo into Roy's Down Air spike perfectly. Just throw him up in the air, and then jump it. Wait until fox is a little bit Above Roy's head and then use Downair, it'll spike him down, though it can be meteor canceled. Only thing you're trying to do is get him low enough pass the stage not to use his illusion to get back to the edge, after that, you counter his Butt Fire (Firefox) And it should send him away, Edge hog, or follow up with a forward smash.
Next, Fox usually uses illusion to get back to the edge right? Now, if you use a down tilt and tipper it, it'll pop fox up just high enough to get a forward smash in, which usually kills all chance of him coming back with proper recovery.
Stuff that you need to know against Fox
1.) If he shines you just 1 TIME when you're off the stage, you're dead. Shine spike is very effective against Roy since he slides like mad from the shine move.
2.) Foxes move very fast and will keep you on your toes with a barrage of attacks, Play defensive until you've got him on the run, and if you're even, try to lure him into attacking and wave dash to forward smash.
3.) At around 90%, Watch out. A up smash from Fox can kill you, as well as him throwing up up and using an Uair, play defensive when your damage starts to get high, UNLESS you're fighting on dreamland.
4.) Don't ever hang on the edge when you're not invulnerable. Keep jumping up and down and grabbing the edge, foxes will shine spike you if they notice you don't know what you're doing.
5.) Last but not least, foxes will **** you if they feel you can't stick with them and will be attacking non stop and wave shining and moving so you can't shield grab. Keep on the move.