: Your protagonist needs specialized information (e.g., translating an ancient text or understanding a complex chemical formula). The only person who knows it is a disgraced former teacher living in a dive bar.
Instead of a literal interpretation, think of the "Hard-Boiled Professor." This is a character who has seen too much, cares too little, and operates in the gray areas of morality.
: A "mature" teacher whose methods are borderline abusive and ethically "sleazy" in pursuit of perfection.
: Smells of stale coffee and old books; wears a tweed jacket with frayed elbows; has a cluttered office tucked away in a basement.