Who is Thomas Callahan from The Pelican Brief and what is their importance?

Thomas Callahan is a popular constitutional law professor at Tulane University, the 45-year-old Callahan is an alcoholic who, therefore, schedules no early classes. He affects a "liberal-chic-academic" look. He has a "history of closeness with female students." Never before has he fallen in love as he has with brilliant and beautiful Darby Shaw. Unlike his Georgetown University classmates from 20 years ago who are working 70-hour weeks in "pressurized law factories" (as he tries for six months after school), Callahan sleeps late, works five hours a day, and his $70,000 a year pays for his bungalow, Porsche, and liquor.