Berkeley's a charming, gallant Englishman who's a buddy of Denys and becomes a friend to Karen when her husband's off gallivanting around.
He's a minor character in the film, whose most interesting quality is that he becomes secretly romantically involved with an African woman, much to the surprise of Denys and Karen. Berkeley dies young, a victim of blackwater fever, a serious complication of malaria.
The takeaway? He epitomizes both the attraction and the risks of life in Africa.