Kingdom hearts is a video game franchize started in 2002 by Tetsuya Nomura and co-created with Squeare Enix and Disney first released on the PS2. The franchize follows the story of Keyblade weilders as they travel to different disney worlds to find friends, fight evil, and protect the relm of light from the powers of darkness. The games have grown over the years expanding the casts of characters, worlds, and lore that has been created.
Yes! There are many different disney characters that are in Kingdom Hearts most notably Goofy, Donald, and Mickey Mouse. In the first game and beyond Goofy and Donald join the game as sidekicks to the main character Sora and they travel to differnt worlds and fight togeather. Mickey Mouse is also a major player and a keyblade weilder throughout the series along with being the king of his respective world. Mickey is not seen much in the first game but in later installments he becomes a major ally and gratly impacts the story and battles as a whole. You could say he's Epic!
Many Final Fantasy characters including Cloud, Cid, Yufi, Squall, Aurron, Sepheroth and others make camios and are major players on their worlds! Sadly none of them are playable but there are tournaments where you can fight the final fantasy characters and optional boss fights where you can go toe to toe against Sepheroth if you want to! I did it myself and it took me three hours fighting Sepheroth to finally beat him.
Yes and No. With the many games and characters that tend to look extremely similar it's easy to see how people on the outside of the fandom are confused but if you play the games in the order they came out it's not confusing at all. Mainly what you need to know is Darkness is bad and light is good. Mainly Kingdom Hearts is about friendships. The main characters are always trying to keep their friends togeather or find lost friends. The villains are always trying to isolate or make one of the friends fall to darkness to achieve power. If you see friends in any sense they are the good guys. Anyone trying to harm the friends or drive them apart are generally bad. If you want to learn more there are a bunch of story run downs on youtube that explain it in a short amount of time here's one of them. (This video was made before Kingdom Hearts III came out and the subsequent DLC's and future games. But it still has a ton of the story to this point.)