English sugar merchant, whom Southey met in Portugal during his visit of 1800–1801 and again in Liverpool in 1804. Koster lodged in Keswick in 1815–1816 after suffering heavy financial losses and later relocated to France, where he died at Bordeaux. Koster’s home in Lisbon was a meeting place for those interested in the arts and sciences and he was a man of wide interests, a member of the Portuguese Royal Academy of Sciences and a writer on economic matters, including A Statement of the Trade in Gold Bullion (1811). His son, Henry Koster, was also a friend of Southey’s.