Lavinia Walsh is an agent with the DIA.
As we get introduced to her in the first recorded adventure, she is working for the FBI but as her career progresses, she will see her employment shift to the far lesser known, and much more secretive Pentagon-based DIA. Even then, while she carried DIA credentials and was paid by that agency and listed as one of their operatives, she truly answered to only one person, the President.
As the sitting Chief Executive tells Walsh in the Oval Office when she told Walsh about the position, "You'd be reporting directly to me. It's a strange arrangement, I'll admit, but I feel like I need someone I can really trust in the intelligence community. I'd like that to be you." This offer of work comes as a result of the events in the first adventure.
Walsh is the daughter of an American mother and a Canadian father, the latter being an investigative reporter for many years until his wife died at which point he became mostly a reliable drunk; you always knew where to find him, just not which bar stool he would be sitting on.
Walsh came to the FBI at a very young age compared to most agents. "At nineteen she'd been recruited by the FBI in a special program designed to increase the number of women in the intelligence community. It meant that while she was at college she trained on weekends and over the summers she worked through a rotation of government jobs. Each meant to contribute to a foundation of knowledge to prepare her for life as an agent. On her twenty-third birthday she graduated to full agent status."
It would only be a year later that the strange events of that first adventure would take place, the conclusion of which would see a 24-year-old Walsh working directly for the President.