Kara was Brendan's ex girlfriend and not the nicest person. She is a manipulator, and she and her group generally look to mistreat people.
In the film, she convinces Dode to sell the fact he saw Brendan over Emily's body to The Pin.
In the film, the money concept is largely glossed over, and it instead looks like it is being done simply because she is very, very spiteful.
However, in the original novella, it's clearly about the money (as even after Dode's death, she tries to blackmail Brendan):
"I still know what Dode was selling, or have you forgotten? But I'd
play it smart. A 5 8 quick call from a payphone to copland and you're
She came very close to me. Her robe fell open slightly.
"Five thousand. Cash. I know you can get it from the Pin, but even if
you can't I want it by first period tomorrow, or I play my hand and