I don't know Japanese, but it always helps to have a local or worldwide high-score board. Competition is everything

Yes, competition is important.
I'm not planning to keep an online ladder or anything, BUT the player's performance stats will be kept and shown in a hopefully fun way, so the player/student knows where he/she stands compared to the former him/her. The stats will be kept in a SQLite database. I played with the
sqlite4java library, it works like a charm.
In the same chord as spaced repetition algorithms, I need my game to challenge me on the things I keep forgetting, and less on the thing I know well.
Also, people completely new to Japanese like you should be able to play it/learn with it. So every time a new Japanese character is introduced, it will be shown first.
I really should practice my katakana...
Katakanas are on the way.
I added the 2136 jouyou kanji last night in the game's database, along with their "on" and "kun" readings, plus their English meanings.
Next I must code the logic to choose the right ones for the player's level and current state of knowledge.