Kingdom Hearts is a bizarre, arguably paradoxical, might-be-might-not-be tame series. It was a simple, highly unfitting crossover game in the early 2000s between Final Fantasy and the Disney movies starring a mediator original character, Sora, who looks and fights like a Nomura (FF7-on) but has a clearly visible silly side and child-likeness to better fit in with Disney's stars (barring the fact that Sora is anime and his cohorts are cartoon...what is a boy to do?). In Kingdom Hearts, FF characters were side characters who gave advice and help to Sora and were also available to fight in arena matches; FF icons, mainly the Moogle, made their way into the game; FF magic spells were part of Sora's (and Donald Duck's) skillset; and an FF-style paradigm of light-versus-darkness was made paramount in the plot. That's FF's part of KH. Sora's allies are Donald Duck, who for KH is an outlandish Black Mage, and Goofy, who for KH is an outlandish Warrior (no swords though, just shields). All the core levels were straight out of hit Disney films: the jungle in Tarzan, the town from The Nightmare Before Christmas, and more. Each of these levels appeared to be different planets, which you needed to reach by means of a spaceship which you had to build (it's halfway between LEGO and tinker toys). But what no fan of either company's serieses can deny or defy is KH's battle system: not-turn-based. Sora runs, jumps, and swings his weapon like a cross between Mario and Link, but there's still a little FF-like battle menu in the corner of the screen at all times. Speaking of weapons, Sora was the chosen wielder of a special weapon: Keyblade. Swings like a sword and has magical properties, but looks like it was stolen from Walt Disney World! What's more, it transforms to fit the different movies you've beaten as well as a couple of your FF pals.
These things made the first game was so successful, the story continued, and the series was born.
Like the game's hero, the original game's original villain, Ansem the Handsome, was also from neither but FF-styled; KH2 sees post-pubescent Sora return to face entire scores of FF-styled-but-totally-unique (and interesting) characters on top of even more Disney film worlds and Final Fantasy support characters, and, and, and...it all goes uphill from there.
...but for God's sakes, do you really have to cram Winnie the Pooh into every damn one!?
Related: Final Fantasy series, The World Ends With You