Who is Gen. John Nicholson from Flashman in the Great Game: From the Flashman Papers 1856-1858 and what is their importance?

Flashman served with Nicholson in Afghanistan fifteen years before his arrival in India in 1857. When the two men meet, Flashman is on his way to Jhansi, and Nicholson, disguised as a Baluchi robber, is on his way to Peshawar. In the end, Nicholsondies at Delhi.