This is the second time it happened. Most thought the leader of Akatsuki was Pain but it turned out to be Tobi. I like Tobi ever since I encountered him. But I never expected him to lead a notorious group.
What a twist! It seems Japanese writers / cartoonists want to surprise their readers most of the time and -- adhere to this "Never judge a book by its cover!" So, never judge an anime bad guy by its looks. Additionally, never judge a book while its covered.
Yammy is the strongest when in released form.