Who is Pieter Rousseau from Rendezvous with Rama and what is their importance?

Sergeant Pieter Rousseau takes part in several initial missions into Rama. However, his skills as an expert in telescopes are put to better use when he uses his talents to track the creatures evolving on Rama from his outpost at the bottom of the stairway. His position allows him to issue critical warnings to the explorers' teams when the conditions on Rama change unexpectedly.