You may well know Lakshmi Mittal. His company is responsible for 10% of world steel output. He lives in a 12-bedroom London mansion with butlers on call and a Picasso on the wall. And he famously dropped $55 million on his daughter's wedding, which included a party at Versailles. Few outside the steel industry, though, know much about Aditya and the outsize role he plays in his father's success. Where the 56-year-old Lakshmi is the steel industry visionary, Aditya is the financial dealmaker. The smooth, Wharton-educated son pushed for the $38 billion takeover of Arcelor, the giant European steel company, then helped hammer out every aspect of the deal that closed last June.