Tetsuya Nomura's Kingdom Hearts series is a story about togetherness, hope and using those ideas to overcome the darkness in our hearts. The hero of the series, Sora, is the vessel in which all of these ideas are proven effective. Without him, the series ironically loses its own heart as it's his childish wisdom and tenacity that keeps everyone else in the series striving to be better. However, there was almost a time where that wasn't the case.
Kingdom Hearts is a unique series because it was a once-in-a-lifetime opportunity to mix Disney properties with concepts and characters of Nomura's other series, Final Fantasy. The finished product is a property that maintains the essence of what makes Disney great but places its heroes and characters in situations that aren't likely to be seen in one of its theme parks. During its early phase, both Disney and game publisher Square Enix had their own ideas over who should lead. Disney was championing Mickey Mouse's longtime friend, Donald Duck, as the series protagonist, while Square Enix preferred Mickey and the iconography he brings. On the other hand, Nomura presented another idea: Sora.
Sora was Nomura's compromise between the three parties as he wanted to have a totally original and human character in the lead role. His initial designs included a tail and chainsaw for Sora's weapon. However, Square Enix felt that the tail made Sora resemble another Final Fantasy character, Zidane Tribal, while Disney wasn't interested in a chainsaw as the main weapon. So, in a night, Nomura reworked the design to appease all parties, and Sora was born.
In the original Kingdom Hearts, Sora still retains some influence on Square's interest in Mickey Mouse through his outfit. Sora's yellow shoes, red pants and white gloves are a direct reference to Mickey's design. While he never wears the same outfit in later games, the homage is clear. Even Mickey's keyblade is just a color-swapped version of Sora's, showing how closely tied the two characters were in their conception.
Being human was another important stipulation of Nomura's as he wanted to make sure that the character resonates with the audience. Sora has no powers or skills at the start of the game. By Kingdom Hearts III, all of the abilities he earns are through his own perseverance and strength. This was a deliberate choice as Nomura wanted the players to feel that, like Sora, they can achieve greatness no matter how impossible it may seem. Sora later inspires other characters in the game in the same way, most notably his friend Riku who found his way back from the Darkness.
While Sora wasn't a part of the initial plans for Kingdom Hearts, Tetsuya Nomura continues to make sure that his inclusion in the series hasn't been for nothing. Since his first appearance, he has gone from a character inspired by Mickey Mouse into a hero with the power to inspire millions by being nothing more than a good person. In hindsight, it's clear that the franchise would've never been the same had Sora not joined Kingdom Hearts.