Who is Captain Peter d'Alembord from Sharpe's Christmas and what is their importance?

Captain Peter d'Alembord is one of the men under Sharpe's command at Irati. d'Alembord believes Sharpe to be the best soldier he has ever known. and will ever know. When Sharpe send d'Alembord to offer surrender to Gudin's retreating garrison, Caillou murders Nicholls..... angering both Sharpe and d'Alembord.