While Bad Badtz-Maru could be a villain in Hello Kitty Online, it looks like he will instead be a Country Leader and quest giver. I am not sure if you will be able to join the XO clan, but you may be able to gain faction with it, depending upon how faction and reputation are implemented.
I am personally hoping for a roleplay aspect that is sorely lacking from WoW: that of being able to choose a side by actions instead of during character creation. By choosing to complete quests for Badtz-Maru and his friends, you may end up not being able to complete quests for Hello Kitty and My Melody. There is a small scale version of this when choosing Centaurs to befriend in Desolace, but it does not affect how any of the major NPCs react to you.
Regardless of his role in HKO, he is one year older today, so Happy Birthday Badtz-Maru! May you realize your dreams in the Flower Kingdom and beyond!