From what I understand, the reason Mario Tennis had so long a development cycle was because half way through they didn't like the direction the game was headed so the scraped it and then started over from scratch.
To answer your question though, Thresea is just the Japanese name for Boo, so they are saying that Boo is a playable character. I have been told that Thresea is a play on the english word "terror," but I'm not to sure about that.
Edit: Darn, Bill you beat me to it.