Cassidy is actually the grown-up equivalent of Hat Boy from High Score Kids. The reason she was called Hat Boy is because no one ever asked her name as a kid, and thus never discerned her actual gender.
Joel encountered her as he went to the college on Hydrox. He instantly recognized her, but was confused by the fact that she now had breasts, smelled like flowers, and was not, in fact, a guy. Six months after Joel went to college, he brought her back to the Research Facility where she stayed with Joel and the others, even though Joel knew she'd probably be hated by everyone. By "everyone" he meant the cast or the fans/viewers/stalkers/Wiki guys. Either way it's just a gag.
In Just Passion Through, it was revealed that she went to college for 7 years, mainly for the social aspect. This would place her age in the mid-to-late 20's, depending on how early she entered college. (25 at the youngest.)
Cassidy was the first female character to be voiced by a female voice actress (the mysterious Spaces, who has since resigned from her duties). She was also the first reoccuring character not to be voiced by Matt Wilson (although she is now).
As of 10/22/05, Spaces has resigned from the position of Cassidy's voice. Cassidy's voice now sounds similar to Elly's voice just before the voice change in The Day The Earth Did Stuff (both were voiced by Matt Wilson).
On April 10, 2006, Cassidy was revealed to be the Cloaked Figure. Had the series continued, it would've been revealed that Cassidy had a huge obsession with Phil, and tried to get anyone she viewed as a threat to her relationship with him out of the way (hence the fact that she murdered Rya and, in Strife and Rocket Jump, would have kidnapped Elly and tried to kill her).
- Her eyes are apparently between ocean green and light blue, judging from Depart's All Yours.
- Cassidy is one of only two main characters who has not died yet, the other being June.