Walter P.J. Droege is far from a global household name, but at its peak his vast fortune has made him the 146th richest person in the world. The 71-year-old from Dusseldorf, Germany, launched an investment firm called Droege Group with his wife Hedda in the 1980s. The company now works across 30 countries and makes around $12 billion (£9.2bn) a year in revenue. He does not have children, and has spent some of his wealth on contemporary art – both for himself and to inspire his staff. Droege is currently worth $3.5 billion (£2.7bn).