Meslissa is Spencer Hastings's Little Sister and Melissa is a Widow of Ian Thomas.
She was pregnant at one point with Ian's baby but she lost the baby.
Melissa isn't a main character but you see her alot in the series.
At one point in the show the girls suspected Melissa was -A. Melissa was the Black Widow in the Show because she was blackmailed by -A to do it or they would tell everyone about her baby.