[Shadow5YA]

Spoiler for Kingdom Key's true owner:

Riku was just supposed to have the Kingdom Key because of a formality. Riku touched the Keyblade of a Keyblade user while Sora didn't. Terra came first to Destiny Islands, saw potential in Riku, and performed the ritual with Riku. Aqua came to Destiny Islands last, saw that Terra already gave someone the ability to use a Keyblade, and decided not to give one to Sora because one was enough.

Multiple keyblades between friends meant heated competition between friends that could possibly separate them like Aqua and Terra were separated, and Aqua didn't want that.

However, circumstances changed, as Riku's heart became unstable tainted by darkness, so Sora received it instead. Whether that is because of Sora's connection to Ven, a Keyblade user, or because Sora is special himself, or both, we do not know.

Spoiler for Ven and Sora:

However, that does not mean Sora's heart is not special. His heart was the one that mended Ventus's when it was split into two by Master Xehanort. At the end of Ven's story, Vanitas merges with Ven's heart again, so when Vanitas died, Ven's heart was broken... again.

In part of the Last Episode's ending, we see Sora speaking to Ven's heart. Ven expresses his gratitude for Sora giving him a second chance and says that he will have to sleep again. Then we see the heart go inside Sora. That means Ven's heart returned to Sora after Vanitas's defeat, which is why his body won't wake up.

Ven's story explains why Roxas looked like Ven and how he could use a Keyblade, despite being a "Nobody": because Roxas had Ven's heart. When Sora stabbed himself in KH1 to release Kairi's heart, Ven's heart was also released, and that's what formed into Roxas. Of course, because of Ansem the Wise, Roxas returned to Sora in the end, so Ven's heart is back with Sora.