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The rival archetype rebuilt around temptation, regret, and finally a hard-earned emotional adulthood.
Riku works because Kingdom Hearts lets him fail in ways that actually matter. He is cooler than Sora on paper, but the character's real strength is that he has to claw his way back from being consumed by his own edge.
Example
Across the series, Riku evolves from jealous rival to one of the most stable people in the cast, which is no small feat in that universe.
Why it matters
Riku matters because rival characters stay interesting when they are allowed to mature instead of loop forever.
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