Melissa Courtney has always been in quite a hurry to grow up. All the girls struggled to forge individual identities, and because she happened to end up making friends with the "popular" kids when they all started elementary school together, Melissa became "the cool one." As a teen, Melissa automatically liked whatever was popular. It frightened her to hear a psychologist predict that the seven famous identical children would likely have trouble adjusting socially: in her eyes, the others were ending up just like that. Most of them didn't care what other people thought of them. They couldn't even recognize the latest pop songs! Her sisters showed varying degrees of discomfort with her rebellious attitude, mostly because she seemed driven to it because they embarrassed her. Naturally this lead to hurt feelings: the most outspoken Septs (usually Mary, occasionally Molly) fought with her because she antagonized or put down the others, but Melissa felt she was only defending her unique image. Surprisingly, the others will tell you that Melissa and Mary are the most alike of the Septs because they're both outspoken, loud, headstrong, and opinionated, but as their opinions were often polar opposites, this made things messy.
As the Septuplets have grown, however, Melissa has become more self-confident in her image. Having shown everyone exactly who she is, she no longer frets that she has the same genes as geeky kids. Now that they don't feel resented, the others see Melissa as the boldest of the Septs, and the strongest in standing up to struggles, which they respect. Moreover, since Melissa does share many things with her identical sisters, she has come to understand that they faced the same struggle for individuality that she faced. Knowing this, she'll defend them and their quirky ways if anyone dares to say an unkind word about them in her presence.