Erin Toner is a former Petty Officer who served on the U.S.S. Eisenhower who joined the Navy in hopes of finding a husband for herself.
However, she later seduced Petty Officer Ted Martínez into helping her plan a robbery of the ship's safe which they later did with the unwilling help of Lieutenant Mark Schilz who helped them carry out the crime.
After he had completed the task, they both killed him, dumped his body in a mailbag which they then dragged through the ship before placing it into a cargo pod.
They also framed Schilz for the crime, damaging his reputation to such a degree that he got a dishonorable discharge despite the fact that he was dead. It was also due to that status that his wife and their young daughter did not even get his death benefits which presumably happened after Toner had been arrested.
Toner later killed Martinez, presumably to stop him from going to the authorities with NCIS later finding evidence suggesting that she was guilty.
After being taken into NCIS custody, she was jailed for her crimes.
Toner is extremely manipulative and more than willing to use her good looks to get others to do things for her.
She has grown so used to her luxurious lifestyle that she refuses to spare any thought for Lt. Mark's Schilz's family, instead sarcastically saying they should play the Lottery.
She also expresses no remorse or regret for what she did, implying she is a sociopath.