What's in a name?

Katakana: ソラ

The Japanese word for "Sora" has many meanings in translation. In the case of Sora's official written name, it is presented in katakana in order to remain ambiguous in its meaning. However, "sora" is definitely a Japanese word, and can be written in different ways.

Kanji: 空

When written in kanji, as a noun, it means either sky or the heavens, and is read as sora. The meaning of sky is quite emphasized as the closest to what Sora's actual name might be. This corresponds to Kairi's and Riku's name, which mean sea and land, respectively. This coincidence was intentionally created in order to symbolize the connection between the three friends.

However, another meaning to the kanji is kara, which means empty. This is quite ironic, in that Sora has, on two separate occasions, become "empty". His first would be the time he became a heartless; he had unlocked his heart, losing it to the darkness, and essentially becoming an empty shell. The second occasion is when Sora produces his Nobody, Roxas, who (supposedly) has no heart.

Hiragana: そら

As an interjection, it can mean "Hey!" or "Look!" when written in hiragana. This part is sort of insignificant and irrelevant to his actually name, but I thought it would be interesting enough to mention.

Tetsuya Nomura has a knack for having the name of his characters nature themed. His male protagonists usually have names related to the sky, weather, or heavenly bodies. For example, Squall (FFVIII) is a violent storm. Tidus(FFX), used in Okinawan, means "sun", and of course, Cloud(FFVII).