I'm gonna write a little guide to KH II, not a complete walkthrough, but only somethig to help, kay?
-WARNING - MAJOR SPOILERS-
At the start of the game, you'll be playing as Roxas, a boy from Twilight Town who, for some still unknown reason, has the power to wield the keyblade. This little start is really boring, you just have to do some minor stuff like working to earn munny and a lot of running arround town with your friends, Hayner, Pence and Olette. In the second day, you appear in a place just like the start fo KH1, with the vitrals and evrything, where you have to battle a giant Nobody, instead of a giant heartless, it's really easy if you use all reaction commands.The four friends have the habit of sitting in the clock tower and eating sea-salt ice cream, somethin that who has played 358/2 days will recognize. Also, the separation in days, and the whole Roxas part are really similar tho that game. One time Roxas falls off the clock tower, and talks to kairi, who's at Destiny Islands, trying to remember about Sora, who she has forgotten, just like everyone else who knew him forgot.
You four have only one more week of summer vacation, and are looking for something to write an homework essay about. you'll go check the "seven wonders", have to win a struggle tournament and fight only a little bit. It's all easy. the interesting parts are the dreams that Roxas have, all of them about Sora, and his memories, and the parts when Diz and a guy who calls himself Ansem seem to be planning something. Then, Naminé appears in various ocasions, and tells Roxas that he was never meant to exist, and reveals that he is a nobody, a part of Sora. Axel, another nobody which, if you played 358/2 days or chain of memories, you are already familiar with, appears, and tries to tell Roxas that he was once his best friend, in the organization, and also tries to convince him to go back with him. Before Roxas can answer, time stops, and Diz's voice tells roxas to go to the haunted mansion you already visited, in your search for the wonders. In there, you find Naminé again, who tries to tell you something, but Diz does not let her. You find out that Roxas is trapped in a unreal twilight town, created by Diz. You have to fight Axel, but you suddenly gain another keyblade (Oathkeeper and Oblivion) and an immense strenght, easily winning, and apparently regaining some memories, as you recognize Axel once more.
Then, you continue ahead and enter in a room where you find Donald and Goofy, asleep, and then, Sora. Diz appears and says that you hold part of Sora's power, and that you must join with him, so he can awake. Roxas fights against this oblivion of himself, but then accepts, and joins Sora.
The next scene is Sora's awakening, and his reunion with Donald and Goofy. You three go out of the mansion, talk to Roxas's friends, who have forgotten about him, and find out that the king is at the station. You meet with him, and he orders you to take a train. Hayner, Pence and Olette show up to say bye, and Sora cries, probably because the Roxas part of him is sad to leave his friends. Then, they enter on the train, and go out of Twilight Town.
Sora and Roxas, and Axel.