I really like how the card is at the same time very simple, but also interesting to play. I think it's balanced power-wise, and would make a good uncommon (or rare) in any set. It might be a little bit weak, though; remember your opponent doesn't have to block, so this will often be worse than River Boa or Dryad Sophisticate. Still an excellent Uncommon for a set that can get tricky with pump-spells and provoke-abilities, two quite common things in green. I love it!

edit: maybe change name and type. I think Katamari look like
http://img181.imageshack.us/img181/6172/katamari4sz.jpg (though I also found hentai and Kittens when searching for "Katamari" via google, but those are basic elements of the internet, so I ignored it).