The alternative to the accountant will be a military clerk given special leave to help the Manston Police Department.
You'll have the option to choose between them depending on what you do during the day and where you visit.
One of my favourite games was Mass Effect 2, in which having different people in your team affected the story at times. I'm taking inspiration from this with my novel, as you build your investigation, you'll be able to choose different characters to help you, and they'll have different skills.
Uncovering what exactly has been going on, and stopping it, will require a combination of brains and brawn. Choose wisely :)