Roy started as one of Raptor's slaves, but became a pirate under Pan and then a member of his rescue group. He is also the one who finally kills Raptor. Let's see whom he can best in combat.
Can: Legree
Legree is a slaver and a big racist, as well as the leader of the Burning Cross. His biggest feat is taking a Hot Shot in the head to die. Roy, among other things, survived Kalifa and chased an armed slaver all over an entire ship, so he can win.
Can't: Gaisborn
Gaisborn is the Chief of CP8, who knows all the Rokushiki and wields an axe with terrifying efficiency. He is able to cut down the armored troops of Biomeka with ease. And that's all before the Soldier Hound serum, which made his power ten times greater than before. All in all, he is way above Roy.
Can: Manofly
Roy knows this criminal well and has a low opinion of him. Manofly might have a creative arsenal consisting of various ammunition, but Roy is much more skilled and knows actual Fishman Karate. Besides, one is a warrior and the other a thief.
Can't: Oceanus
Roy might be a Fishman, which makes him strong and great underwater, but Oceanus is a tad above him, as he actually is seawater. He is immune to physical and water attacks, and his skills are superior to Roy's. Roy doesn't stand a chance.
Can: Emily Young
Emily is a cook of Stormwolf, and a competent heroine. Her magical frying pan, her unlimited boiling oil and her Fire Eggs make her a threat to low-level enemies, while her wits allowed her to kill an enemy way beyond her level, a feat she shares with Roy himself. Yet Roy would win in a fair fight. He is too much for her underwater and his Fishman Karate will stop all her heat-based spells due to hard counters.
Can't: The Tight Flower
Feel free to laugh at the name, this is a monster of the United Alliance of Terror. Roy has one big advantage, and that's mobility. He can run circles around the floral menace if he wants to. The problem is, he has the elemental disadvantage and much less firepower. The Flower can shrug off his mediocre water attacks and pelt him with its explosive seeds.
Can: Streit
Streit is one of the minions of Don Krieg, who wields twin bazookas. While he is strong and has great firepower, Roy is agile enough to dodge all the attacks, can counter with his water and will easily win in close range due to his fists and trident.
Can't: Kallie
Kallie is the one Stormwolf member that will destroy Roy underwater. She can hold her breath underwater for two hours, is agile enough to dodge his trident, has the very strong Super Net and can use Ice Eggs to freeze him before impaling him with her harpoons.
Can: Charlotte Flampe
Roy will have to fight the Big Mom Pirates once more if he wants to save Pan, and he can handle a few of them. Flampe has her blowdart, and she can definitely deal damage from afar, but in close range, Roy is simply much more experienced. His water shots will block the blowdarts and his trident will land decisive melee blows. Also, he lasted longer against Kalifa than Flampe did.
Can't: Raptor
To be fair, Roy did kill Raptor by stabbing him in the heart. But most of the fight was done by Virago, and it took teamwork to beat Awakened Raptor. The Fruit user, while he would struggle a bit with the Goldfish Arrows, is fast enough to avoid every attack, can fly, has Haki and is a master swordsman. Overall, he has everything above Roy, except friends.