The Idolmaster (Series, Movie, and Art): Ami and Mami Futami

Ami and Mami are a pair of mischievous twins who like to dress up, and play jokes on their friends.  Both are spirited and full of energy, although Mami comes across as slightly more calm and mature.

Originally, both would take turns performing under Ami's name a fact known only to the members of 765 production.  The easiest way to tell them apart before Mami grew her hair long, is that Ami has her ponytail on the right side and Mami on the left.