I would argue that the two most important leadership qualities shown by de Klerk were flexibility and courage.
De Klerk had to be flexible because he was dealing with a number of different constituencies pulling on him in different ways. For example, before he became president, he had to be flexible enough to be very conservative in his running of the Education Ministry. This was necessary so that very conservative people would not leave his party.
At the same time, de Klerk clearly had courage. As soon as he became president, he started to work to end apartheid. This took a great deal of courage because the system had been in existence for a very long time and abolishing it was very unpopular among some segments of the population.
In these ways, de Klerk showed that he had the courage to fight for what he thought needed to be done, but also the flexibility to moderate his actions when necessary.