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Kingdom Hearts isn't that confusing and I'm gonna prove it by summarizing every game in just one tweet each. Go.
KH1: a boy named Sora is the chosen one of a mystical weapon called the keyblade. His friends go missing and he travels through Disney movie themed worlds to find them. One of his friends, Riku, is manipulated to help the antagonists, but he comes around. Sora beats the bad guy.
CoM: Sora and friends are lead to a castle where a mysterious organization is toying with him. The organization manipulates Sora's memories without his knowledge, but are eventually defeated. Sora goes into cryo stasis for 1 year to have his memories repaired.
KH2: Sora wakes up and the organization is now trying to build a synthetic version of a magical entity known as kingdom hearts. They manipulate him to unknowingly help build it, but he eventually defeats them regardless.
Days: You play as a member of the organization with ties to Sora. He slowly grows dismay for the organization, except for two friends. One of his friends is revealed to be a synthetic person, and must be killed to heal Sora's memories. Roxas kills her then leaves the organization
BBS: a prequel to everything so far. An evil man, Xehanort, manipulates 3 friends to help him wage a battle to summon a WMD. He fails, but lives on by possessing one of the three friends.
Coded: two chipmunks put Norton Antivirus (modeled after Sora) in a book (modeled after his friend Riku) to get rid of viruses. Norton Antivirus is gay for the book. No this game is not particularly important to the plot.
DDD: Sora and Riku perform a test in a dream world to prove themselves, but the test is hijacked by a reformed organization built by Xehanort, as well as his time traveled other selves, and those he has possessed. Sora is nearly possessed, but is saved, setting the stage for KH3
KH3: Sora works to gain a power necessary to save the protagonists of BBS. he succeeds and they wage a battle with Xehanort and his time travel squad, who is once again trying to make a WMD. Xehanort succeeds, but Sora and his friends just barely stop him.
KH Chi: in a time where mostly everyone had keyblades, a war is slowly brewing. This war would claim the lives of pretty much everyone, but a group known as the Dandelions jump to another timeline to avoid the war altogether.
Chi Back Cover: five master keyblade wielders are tasked with leading the people in KH Chi. They are slowly split apart by paranoia planted by their now departed master. They eventually all fight, leading to the keyblade war of KH Chi.
Union X: Takes place in the timeline after the Dandelions jumped away. They lost their memories of the time jump, but the war does not happen. Still, annihilation is looming, and five young keyblade wielders try to save their new world while investigating who is responsible.
Re:Mind (haven't finished it but here's what I got so far): Sora does time travel to save his friend from dying. The price he pays is being unable to return to the physical world. Riku, his boyfriend, looks for him. a data copy of Sora fights some computer boss fights.
The only actually confusing parts of kingdom hearts are small details like "why does vanitas look like Sora" or "how does Xion work, like, mechanically in-universe." The explanation of some stuff could be better but the raw plot is very straight forward
Grand finale: explaining EVERY game in one tweet:
Anime boy fights men with very good voice actors. Friendship is very strong. Hearts hearts hearts.
Anime boy fights men with very good voice actors. Friendship is very strong. Hearts hearts hearts.
