Miller and Murphy are two agents from the special US Treasury Task Force. They serve as an example of the classic good cop/bad cop trope. Introduced in book one, Off to Be the Wizard, Miller and Murphy first make an appearance investigating Martin concerning suspicious activity involving his bank account. As they explain, it is not the amount of money that first catches their attention, but the number of large deposits that appear from seemingly nowhere.
In book 2, they are approached by Jimmy after his exile, who promises to explain to them how Martin was able to commit fraud and disappeared from their interrogation room. Jimmy walks them through the steps for finding the repository and lowering his magnetic field to normal levels, at which point he disappears from under their noses.
In book 3, they have been working with Todd to find another copy of the repository and use it to fund a new task force. Unbeknownst to them, Todd uses the opportunity to escape from prison.
In book 5, they have been transferred to a new assignment by their boss, Director Britney Ryan, tasked with staking out Jimmy in his new home in Las Vegas, NV. Eventually, Brit recruits Jimmy into the task force as a special consultant and assigns Miller and Murphy as his permanent assistants. Jimmy buys them a luxurious penthouse next to his own, as repayment for all the pranks he pulled on them.
In book 6, after Martin and Gwen are frozen, Miller, Murphy, and Jimmy are sent to the Banks residence to put Mattie and Brewster in protective custody.