She couldn't save his life, but she says he forever changed hers.
"As I walked into the house at 1 a.m. this morning covered in blood, I said to myself, 'This is what I was born to do, help save lives,'" Fisher wrote on her Facebook page soon after she learned he had died.
She knew how serious the 31-year-old's injuries were, but her hope he could be saved was reflected in her determination - and his.
PHOTOS: Remembering Tpr. Sean Cullen
"When I first walked up I asked his name, and I'm just saying, 'Fight through it Sean, fight through it,' and he was fighting," said Fisher. "Trooper is seriously an understatement because he was fighting for his life."
Fisher and her boyfriend, who is also a New Jersey State trooper, drove up on the scene as they were driving home from dinner. They pulled over to see if they could assist law enforcement with a car fire - moments later she heard the sound of another car striking Cullen.
"I wanted to get him home to his family. As we were working on him, one of the troopers yelled out that he had a baby and he had another one on the way, and I think that all pushed us to work even harder," said Fisher.
Fisher's brother-in-law is also a New Jersey State trooper and knew both Sean and his brother, Garrett. Both, she says, have reputations as great guys among their colleagues.
"Please God protect these troopers and all of law enforcement as they go out to protect us. Rest in peace trooper Cullen," said Fisher.
The exact cause of the accident is still under investigation.
Stephanie's boyfriend, who could not speak on camera, says as a member of law enforcement he's on duty 24/7 and what they did wasn't the brave thing, but the right thing.