Who is Colonel Caillou from Sharpe's Christmas and what is their importance?

Colonel Caillou commands the 75th Infantry of France and was awarded the French Eagle by Napoleon for his service. Caillou believes Gudin to be inept and threatens to have him killed if his Eagle is lost during their march to Irati. He also unsuccessfully attempts to persuade Gudin to leave the woman and children behind at the Fort at Ochagavia. When d'Alembord and Nicholls agree to surrender under a flag of truce, Gudin is killed by Caillou.