Here is a list of all the Final Fantasy Characters that make cameos or more important roles in Kingdom Hearts 2:
Aerith...
Cloud's girlfriend from FFVII who was killed by Sephiroth. In KH, however, all the FF characters are somewhat younger then in their respective original games, so she makes an appearance living in Traverse Town with Leon, Cid, and Yuffie while looking for Cloud. In KH2, she is taking part in rebuilding Hollow Bastion.
Auron was one of the summoner Yuna's guardians in FFX. He was one of the central characters to the plot and was probably to person Yuna went to for advice the most. In KH2, he is hired by Hades to take out Hercules.
A master of airships in many Final Fantasy games and the go-to guy for Gummi ships in Kingdom Hearts. A character named Cid has been in almost every FF game to date.
The main character and hero of FFVII. His girlfriend is Aerith [known as Aeris in FFVII] who Sephiroth famously kills. However, since all of the FF characters are famously younger in tihs game then in the games they were originally in, Aerith is able to make an appearance. Cloud was hired as a hitman to kill Hercules in the first KH, and Sora, Donald and Goofy encountered him before Sora convinced him to turn on Hades who had hired him. He is featured in KH2 as taking part in te rebuilding of Hollow Bastion and facing off against Sephiroth.
Known as Fujin in FFVIII, along with Seifer and Rai [Raijin in this game], she is a rival to Roxas, Pence, Olette, and Hayner. They participate in the Struggle Tournament together.
Leon, known as Squall Leonheart in FFVIII, was one of Sora's advisors in a way in Kingdom Hearts. He changed his name from Squall to Leon for this game apparently to distance himself from his memories. In KH, he lived in Traverse Town, along with Aerith, Cid, and Yuffie. For KH2, he is helping in rebuilding Hollow Bastion.
A main character from FFX-2, she has a mysterious past that was discovered in FFX-2, in which she went on a journey with Yuna and Rikku to find Tidus, who was believed to have died...in a certain form of the word. In this game she and the other two are fairies.
Rai, known as Raijin in FFVIII, is partners with Seifer and Fuu [Fujin in this game]. They participate in the Struggle Tournament, hosted by Setzer, and serve as rivals to Roxas and his friends.
A main character from FFX and FFX-2, she was one of Yuna's guardians on her pilgrimage to defeat Sin in FFX. In FFX-2, she set out with Yuna and Paine to find Tidus, who was believed to have died defeating Sin. In this game the three of them are fairies.
From FFVIII, he lives in Twilight Town and along with Fuu and Rai [Fujin and Raijin, also from FFVIII] they act as rivals towards Roxas and his friends. They also participate in the Struggle Tournament.
As if Sephiroth needs an intro. He was the main villain in FFVII and possibly the most famous villian ever. In Kingdom Hearts he was featured as a cameo in a bonus match against Sora in the Olympus Coliseum. In the Final Mix version of the game, he was shown fighting his nemesis and the hero of FFVII, Cloud.
From FFVI, Nomura included him partly because he wanted to see him with newer graphics. He is encountered in Twilight Town and participates in the Struggle.
A lively and always cheerful girl from FFVII. In KH she lived in Traverse Town with the others, and in this game she is located in Hollow Bastion, and helping rebuild it.
A main character from Final Fantasy X and X-2, she, along with her guardians, set out on a journey to defeat a colossal beast known as Sin that had been terrorizing Spira for thousands of years. In this game, her, Rikku, and Paine are little fairies.
