Special Agent Paula Cassidy was an NCIS (agency) Agent who mainly worked from the Pentagon, and also spent some time in Guantanamo Bay as an interrogator.
In Minimum Security (episode), in the file that Kate and Tony were reading while the team were heading to Gitmo, her place of birth was revealed as being Toronto, Ontario - which interestingly is the very same place that Jessica Steen, the actress who played her, was born.
When NCIS Agents Leroy Jethro Gibbs, Anthony DiNozzo and Caitlin Todd arrived to investigate the death of Khalil Sa'id, a Petty Officer who had worked at Gitmo prior to his death, Cassidy was left out of the loop because of five un-mailed letters of hers had been found in Sa'id's personal belongings and as such, Cassidy was a suspect.
She was later cleared although she developed a strong resentment of Gibbs and the others for keeping her out of the investigation. During that time, she and DiNozzo flirted with the possibility of a romantic relationship. Paula was later shocked to discover that Sa'id had been in league with Nassir Al-Jazair and the two had been planning to kill Tariq Abassi, one of Bin Laden's son-in-laws, but Cassidy intervened to stop the assassination, shooting Nassir in the shoulder as she didn't want him to become a martyr.
After returning from Gitmo, Cassidy had brief appearances or cameos in the episodes Bête Noire (episode) and Reveille (episode), where she was seen in Gitmo on a video conference, talking to Gibbs and the team.
She then officially reappeared in the Season 2 episode, Heart Break (episode) where as Agent Afloat on the USS Kennedy, she helped the team with a case regarding Michael Dornan, a Navy Commander who had died after having a successful surgery being performed on him. She later returned to NCIS HQ where she revealed that Dornan had asked her not to file a report on a young Ensign and as such, Gibbs berated her for not revealing a possible suspect and for looking the other way.
During the interim between Caitlin Todd's death in Twilight (episode) and Ziva David being assigned to Gibbs's team in Silver War (episode), Cassidy was temporarily assigned to the team, serving as DiNozzo's partner in the episode Mind Games.
Cassidy's final appearance was in the Season 4 episode, Grace Period (episode) where she lost two of her team members, Agents James Nelson and Rick Hall to a suicide bomber. The aftermath left her emotionally scarred and distraught as she began blaming herself for what had happened, resulting in her developing and experiencing survivor's guilt.
During the investigation, she resented Ziva's attitude and the two were shown to have a hostile relationship with Paula often getting Ziva's surname wrong on purpose.
At the scene, Cassidy discovered that Jamal Malik, the true mastermind was attempting to kill everyone. She then jumped on him with the doors closing behind them despite Gibbs and Tony trying to come to her aid as she and Malik struggled for control of the device.
However, Malik set the device in motion and in her final moment, Cassidy saw a vision of both Hall and Nelson standing alongside her.
Seconds later, the bomb exploded, shaking the foundations of the building while also killing both Cassidy and Malik. Cassidy's death left the team, especially Tony devastated and was responsible for giving him the courage he needed to tell his girlfriend that he loved her.
Despite being an NCIS Special Agent for six years, Cassidy often made rookie mistakes. This was first revealed in the Season 1 episode, "Minimum Security" where she had suspicions in regards to both Sa'id and Al-Jazair but had let them slide yet she later came to regret it.
When Cassidy returned a year or so later in the Season 2 episode, "Heart Break", she ran into trouble yet again when she stated that she hadn't filed a report on a possible suspect, Ensign Evan Hayes because the Commander, Michael Dornan, had asked her not to.
Gibbs was furious at her for doing this and said that NCIS Agents weren't supposed to look the other way. This in turn resulted in Kate being forced to shoot Ensign Hayes when she thought he was about to fire at Gibbs.
She also seemed to get very emotional as seen in "Grace Period" where after losing her team to a suicide bomber, Cassidy began experiencing survivor's guilt and blamed herself for their deaths.
- Like McGee, Cassidy wore a watch on her right hand which suggested that she was probably left-handed.