22 Apr 2010
Literally: Unlock the character "Hu Jul Geun" - (the lady who's pictured in the achievement icon).
I don't know the exact requirements to unlock her, but I'll tell you what happened for me. I completed a 3 song set in superstar (the first mode in the main menu) with the character "Ung-i" (the boy character who is initially unlocked). I then played the two roulette songs afterwards and then a wheel of fortune-like spinner comes up. I spun and got the section that had the "item" label on it. After I passed the next song (with an "S" rating) the character and achievement unlocked for me.
I'd done the spinner once before and NOT hit the segment with "item" on it, and nothing unlocked after I passed that song. I also don't know if the item that gets unlocked is always this character or not. Sorry I can't be more helpful -- just hoping that some information is better than nothing.