Who is Dan Lipranzer from Presumed Innocent and what is their importance?

Dan "Lip" Lipranzer is a police officer and Rusty Sabich's friend and co-investigator. He is an upstanding cop, with no time for colleagues who are bullies, bigots, or on the take. His father was a policeman and Lip left college to join the force after his father died. He has a rheumy-eyed and sulking look and is a loner on the force, but he is also a gifted investigator, skeptical in the way of a good policemen, pragmatic, and not heartless. A scholar of the city's underground life, he is intensely loyal to friends like Rusty, has a dry sense of humor, and is wary of women who have a roaming eye. He is unmarried but sees an older woman on a regular and casual basis.