After the sudden transfer of JJ to Pentagon, the team needed a new seventh member. And Rossi immediately knew who. He arrested Ashley's father, the Redmond Ripper when she was just eighteen and she started training for the FBI, to never be like him.
But the thing is she still wasn't over it. Who would be? She was basically still a kid when she realized her father was a serial killer. She acted fine, but you knew that look in her eyes. She was curious and eager to learn, to catch people like her dad.
She admired you quite a lot as a senior, and she knew that if she asked you would've let her, but she didn't. She didn't ask when she snuck into a file room and started looking for her father's file. She just wanted to read what he said when interrogated, his motive, his reasons.
As you walked by the closed room, you heard papers rustling and decided to check. She didn't hear you walk in, she was way too focused on finding the file labeled Charles Beauchamp to notice.
But you couldn't miss that blonde hair.