(TIME) -- Jamie Dimon, the onetime heir apparent at Citigroup before being ousted in 1998, may yet have the last laugh. Dimon, 48, first engineered a monumental turnaround at Chicago-based Bank One-- pushing out top managers, slashing costs by $1.5 billion and helping to turn a $511 million loss in 2000 into a $3.5 billion annual profit three years later. Then he staged a triumphant return to New York City, when Bank One merged with JPMorgan Chase last year. In 2006 Dimon will become the merged firm's CEO, but he has already begun reshaping the institution in the trademark no-nonsense style he developed while at Citigroup. He has made key personnel changes in the investment-banking and bond areas, brought the bank's IT management in-house and initiated a risk-management review, while instilling his relentless bottom-line ethic throughout the business. It may take some time for Dimon to deliver the promised $3 billion of annual cost savings from the merger. But by melding his old bank's retail, credit-card and small-business strengths with JPMorgan Chase's investment-banking and asset-management prowess, Dimon has turned what some called Citigroup West into a colossus that can give Citi, as Dimon says, "a run for its money." --By Daniel Kadlec