Keyblade wielders, heros, bladers, warriors, users are basically people who has the power to fight with Keyblades. With that being said, there are (were) many Keyblade holders in the history of Kingdom Hearts. Sora, Riku, Mickey, possibly Kairi all fall into that category.
I would not really call them masters but in Kingdom Hearts, Sora was considered a Keyblade Master by Jiminey Cricket and Goofy, also Malificent considered Riku the rightful ruler of the Key, perhaps Riku understood it wrongly, maybe Jiminey, Malificent, and Riku were possibly all ignorant on the fact about how the Keyblades were (are) used, represented, and existed through time.
Since the Keyblades has long lines of holders in history, we can't specify a particular Master. Maybe a person who uses the Keyblade to its full potential and can either save the world or destroy it with the awsome power of the Keyblade could be considered Keyblade Masters.
The UEM in the Secret Ending was amazingly powerful, perhaps there were (are) people like that in the history of Kingdom Hearts who can wield the Keyblades with so much power and be considered Masters, but that is just all high speculation. In the Secret Ending, the UEM turned his Keybalde into a ball of energy and thrusted it into the sky as a heart shaped figured formed. Perhaps that is what it means to be a Keyblade Master.
Did the UEM use his Keyblade's power to bring forth Kingdom Hearts?
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