Bruce Witherell, a former top executive of Freddie Mac, has emerged as a key player in the new mortgage investing real estate investment trust being launched by Cerberus. Witherell, who was a chief operating officer at Freddie from 2009 to 2011, is chairman of Cerberus Mortgage Capital, a real estate investment trust that hopes to raise at least $150 million through an initial public offering. At Freddie, Witherell was in charge of day-to-day operations of three lines of business: single-family, multifamily and capital markets.