Tiara is the second character you unlock (well, the second one I unlocked -- she became available around the time I reached "debut" level with the initial character, Ung-i). She looks like the picture in the achievement but without the crown. To get this achievement you need to score a music score of 100 on three consecutive songs while using Tiara. I don't know if it has to be in Superstar mode or the three songs in a normal set, but that's how I did it.
You don't need to get a full combo to score 100. I'm not exactly sure what the formula is because it's so haphazard, but you can definitely miss some of the notes and still score 100 (I've gotten 100 with 48 misses, for example).
If you don't know a song, don't worry -- the pitch detection in this game is pretty much garbage. Just singing a straight single tone or even putting the microphone in front of a loud fan (especially the latter) is enough to to trigger a full combo in most songs. You may need to experiment to find songs that are more easily achievable -- the three I sang were "Julian," "mo-reu-ri" and "Just Once."