From what I remember, Su is like, really softspoken and gentle right?

Su is optimistic, adaptable, passive and peaceful. She tries to find good about the situation rather change it. Like Snow White and Aurora. So Merida and Jasmine are most different from her. They are both rebellous, risky, combative, somewhat aggressive and not very optimistic.
@princesslullaby, she is the least agrumentative of sisters, yeah.

P.S. No angry comments, please!

I guess.