Vacillating between past and present tense, he describes the drive and ambition of the 30-year-old, who was just starting to live her longtime dream when a gunman entered the unlocked door to her classroom at Sandy Hook Elementary and killed the substitute teacher and 16 of her young pupils.
"She was so nurturing of the children," said Rousseau, a Quebec native who has lived in Connecticut for more than 30 years.
"She has so much patience. She just enjoys being with the children."
In an interview with Radio-Canada from his home in Southbury, Conn., Rousseau described his daughter, her love of teaching and the sequence of events that led to her death.
She was one of six adults and 20 children killed in the massacre that stunned the world and left Rousseau's family with a gaping hole that was once filled with promise.
Gilles Rousseau speaks in a matter-of-fact tone when he describes Lauren's work ethic. A glimpse of his underlying grief breaks through only when he pauses while describing a future – a husband, a permanent job, children – that his daughter will never have.
"It must have been fast," he said, referencing the injuries his daughter suffered as was detailed to the family by the local coroner.
"She probably did not suffer. It is a relief. If she was kidnapped and raped, it would have been much harder to feel the pain of her feeling the pain in the process. This was quick."
Though her father is from Canada, Lauren was born in the U.S. and grew up in Danbury, Conn..
Gilles Rousseau still has family in Quebec, and they are expected to travel to Connecticut for Lauren's funeral this week.
François and Colette Rousseau, Lauren's uncle and aunt, watched in horror as the tragedy unfolded on television last Friday from their home in Quebec's Eastern Townships.
"She was at the wrong place at the wrong time," Colette Rousseau said.
"If she hadn't been replacing the permanent teacher that day, she wouldn't have been there," François Rousseau added. "She could have been in another class."
Worked 3 jobs
Lauren Rousseau earned a master's degree in education, but had been searching since graduation for a permanent job. On weekends, she worked for a catering business and, when she wasn't teaching, she picked up shifts at Starbucks.
Eight weeks before her death, Lauren was hired as a long-term substitute teacher at Sandy Hook. But without a permanent position and benefits, she wasn't making enough money to quit her other jobs, her father said.
Despite the workload, she was thrilled to be teaching. It was a goal she'd had since she was in kindergarten.
"She was very, very happy," Gilles Rousseau said. "She saw it as a foot in the door."
She was in her classroom with her students when the gunfire started down the hallway. As a substitute, she didn't have keys to the doors, which her father said locked from the inside only with a key.
It's through that unlocked door that the gunman entered her classroom after finding the first classroom locked, Gilles Rousseau said.
The bullets from the rifle fire were so powerful, they passed through the school walls and left three holes in his daughter's car, parked in the lot.
It wasn't until 1 a.m. Saturday morning, when a state police officer knocked on the door, that her family got confirmation that Lauren had died in the shooting.
"It took too long to tell us," Lauren's father said. "We knew. By 12 p.m., we knew. All the students and the teachers who were still alive, they left the school. Those who were missing, they were dead."
Details learned through media
Her family learned the exact sequence of events through the media Tuesday, though they'd been advised authorities would update them first.
Rousseau said, despite that disappointment, his family has received tremendous support from the community and police.
They offered to place a squad car outside their home to keep reporters back, an offer Rousseau said he denied, though they have been sparing with their public comments. His wife, Lauren's stepmother, has struggled with the media attention.
"I told [my wife], this is therapeutic for me to talk about my daughter," he said.
Rousseau said he's been able to cope through support from his family. The words of U.S. President Barack Obama, who met personally with each of the families who lost loved ones in Friday's shooting, were also a comfort, he said.
State of shock
Rousseau admits the full weight of the loss may have not sunk in. He speaks candidly about his daughter's injuries, and says that he was advised by the coroner he would not be permitted to see her because of the severity of the damage.
Lauren Rousseau will be cremated before her funeral Thursday.
"I think it will be worse after the funeral, and after the press is gone, and after we're on our own," said Rousseau.
"I was warned about that by many people – it's the aftermath.
"When the limelight is on you, you kind of forget. I think when you're back home by yourself and doing your daily chores, then I think it will hit."