Kristy is one of those bubbly, vibrant, vivacious girls who live life to the fullest—and then some. When Macy first sees her, she's stunned by her energy:
She was full of something so electric, alive, that I recognized it instantly, if only because it was so lacking in myself. (4.58)
Can you say foil?
And that kind of approach to life is just what Macy needs. She and Kristy become instant friends, and Kristy is fiercely loyal and protective whenever someone is mean to Macy.
Kristy has had a hard life herself, with the (literal) scars to show for it. Let's see: her mom, driving drunk, injured Kristy in a car wreck and then abandoned her and her sister. Oh, and her old boyfriend left her for dead in a field: "when I was in the hospital, right after the accident, they thought I was going to die. I was really fucked up, big time" (7.206).
Not exactly the stuff of fairy tales.
But through it all, Kristy has never lost hope, and she always believes that an "extraordinary boy" is just around the corner. Her philosophy, as she tells Macy, is "don't be afraid. Be alive" (7.211). Kristy gives Macy a few demonstrations of her fearlessness, speaking her mind to everyone all the time, yelling at Bethany and Amanda, and even telling Macy's mom all about Wes and Macy's relationship.
In the end, her attitude rubs off on Macy. It seems like the makeover she gave her that first night they went out is finally complete.