Sora can fight Riku on Destiny Islands before reporting to Kairi for the first time. Riku is the most difficult challenge on the island. His attacks are similar to other battles with his normal strikes. He has two special attacks; if Riku gets knocked down, he will jump back up kicking Sora, this can be dodged by jumping out of the way. His other comes if Sora hits him while he is guarding three times, he will unleash a powerful strike though this can be countered or dodged. The difficulty of this battle comes from falling off the small battle arena (if Sora goes out of bounds, the battle ends in defeat). Riku is worth 5 EXP.
His moveset is similar to the battle at Destiny Islands, but now that he has the Soul Eater with him, they can be very damaging. He moves very fast too, so Dodge Roll out of harm's way. Melee attacks are recommended for this battle and simply heal when necessary.
Consider summoning Tinker Bell to restore Sora's HP periodically and to revive him if he falls.
After Riku's first life bar has been drained, he will start using a new attack; he'll spin around while swinging his Keyblade, similar to Sora's Vortex. He uses this attack very often, mostly to break Sora's combos. During this time, try keeping Sora at a safe distance, but not too far or Riku will make a high leap and attempt to stab him with his Keyblade, creating shockwaves that move sideways from his position. Now will be a good time to use Ars Arcanum, but just use it a few times and try saving MP, because Sora will be needing it later.
Eventually, Riku will cloak himself with darkness and a purple aura will surround him. During this period, his speed and agility increase and he gains two new attacks; one involves throwing his Keyblade at Sora, similar to Strike Raid (be careful when countering this attack, as if it hits Riku he will use Dark Firaga) and his infamous Dark Aura. Fortunately, his thrown Keyblade can be deflected either by guarding or striking with perfect timing; for each successful deflect, Riku will receive damage and be momentarily stunned. Seize this opportunity and attack relentlessly.
When Riku's health bar drops to his final one, he'll become more aggressive and uses Dark Aura very often (watch out for the phrases "Behold the power of darkness!" and "Open your heart to darkness!"). Riku's Dark Aura is predictable, but it can be difficult to dodge. When he begins floating and charging, stay a safe distance away from him and High Jump right before he starts shooting around. Immediately execute Glide and move around in a circular fashion. Near the end of his assault, repeat the phase and start Gliding again. Riku's finisher covers about 60% of the arena, so it'll be pretty hard to evade it. Alternatively, when he charges to one particular end of the arena (he usually starts with the top left corner), jump and stay there: Riku will not charge into the same area twice.
Attacking and counterattacking are good tactics while Riku is in this form, just pay attention to what he says and heal occasionally.
Note that aside from the first hit, he cannot use Dark Aura when Sora stands directly next to the computer. However, he will Strike Raid and combo Sora immediately afterward.
When Riku's health reaches critical levels, be sure to finish him quickly, as he will use Dark Aura very often, prolonging the fight. Aside from that, he'll start to keep his distance from Sora, so track him down and continue attacking. A good pattern to get into that causes great damage is immediately after executing a full or half combo, Guard, and repeat, almost never giving him a chance to attack.
Kingdom Hearts II
Sora battles the hooded Riku in a one-on-one match in the Land of Dragons. He can be damaged with the Reaction Command provided by the Rapid Thrusters that surround him. If enough Ethers are stocked, a good way to damage Riku is by using Trinity Limit. The battle ends when Sora deals enough damage to Riku. If Sora is struggling, just battle the Rapid Thrusters since they are endless and try to level up.
Kingdom Hearts 358/2 Days
Riku will block almost all of Roxas's attacks, but will not counterattack. Pursue and attack Riku in order to herd him to the southern tunnels. While several Heartless will appear throughout the area, just ignore them and focus on Riku. Once Riku is chased into the southernmost tunnel, he will escape through a Dark Corridor and the mission will end.
Riku is weak to Aero, and can be brought down from long-range with little risk to Roxas is he has stocked enough of the spell, but make sure not to let them be negated by Guard Counter. Stocking several Cures is also useful, unless grinding Riku for EXP using the Extreme. In that case, Roxas should rely only on Aero to avoid taking damage.
When Riku is low on health, he infuses the Oblivion with shining energy, increasing the range and power of his attacks for the rest of the fight. During this phase, Riku no longer staggers from Roxas's attacks.