Kay Hartley is a Wall Street lawyer and a former law school classmate of Serena Southerlyn. Hartley took a pro bono case involving her aunt, Diane Payton, who killed Diane’s son Danny due to his schizophrenia causing him to commit murder.
Southerlyn assumed Hartley just wanted a career boost with how much stress she was putting on a justifiable homicide defense. In the end, it didn’t pan out, and Diane was convicted. (L&O: "Mother's Day")