Julie Dwyer was first mentioned in Clerks as an old high school friend of Dante Hicks and Randal Graves who died at a swimming pool at a YMCA. Her funeral scene was eventually shown (in animated form), in Clerks: The Lost Scene, which appeared on the Clerks X DVD set.
In Mallrats (which took place the day before Clerks.), Dwyer is told that the camera adds 10 pounds, so she over-exercises before her appearance on the game show "Truth or Date", which leads to her death.
She was also mentioned in Chasing Amy and Clerks II.