The acting director of the Federal Housing Finance Agency and overseer of Fannie Mae and Freddie Mac, Mr. DeMarco is a soft-spoken, career public servant — and under fire. In the thankless job of conservator for the loss-ridden mortgage finance giants, he has a duty to ensure that the companies operate in the best interests of the taxpayers who own them. That means working to keep a lid on the companies’ losses, which now total $183 billion.