Who is Professor Pesca from The Woman in White and what is their importance?

Professor Pesca, an Italian, formerly taught at the University of Padua. Walter Hartright once rescued him from drowning and to repay this debt of gratitude..... Pesca recommends him to a tutoring job to at Limmeridge House in Cumberland. In the end, we learn that Pesca is actually a high officer in a secret society of political assassins.