Who is John MacKeon from Sharpe's Skirmish and what is their importance?

John MacKeon is Tubbs' foreman. He is described as a tall, frowning man with huge hands, a heavy brow and scowling eyes. MacKeon serves with Sharpe at Gawilghur, and he reminds him that there is always a back way in. He then suggests the construction of makeshift caltrops. His idea is hugely responsible for the defeat of the French at San Miguel.