AdventureQuest Worlds. It’s eaten about an hour and a half of my life already, which is why I am putting it down now. As you can see from the artwork, it’s heavily anime-flavored, but not excessively cheesecake.
I generally like Artix Entertainment’s stuff; they have a good handle on the two-tier gameplay model. You can have fun playing their free games, while still getting a decent deal on their pay versions.