"You know," he said, "sometimes if I'm alone, and there's no one around...
He glanced up and down the bar and lowered his voice.
"...I order a sloe gin fizz," he said. - Paper Doll

Lieutenant Lee Farrell is a homicide detective who works for Quirk. He first appears in Paper Doll, where he's assigned to a difficult case. He's openly gay, and as a result gets assigned to most of the difficult cases (as Quirk puts it, the police force can't fire Farrell for being gay, but they can deliberately assign him to dead-end cases). Throughout the book, he suffers severe distress as his lover Brian succumbs to end-stage AIDS. Lee is initially hostile to Spenser's assistance with the case. However Spenser's acceptance of Lee as an equal, and support for Lee's relationship with Brian, lead to their becoming close friends. And Lee demonstrates his professional skill by untangling a financial crime related to Spenser's case which brings down a sitting U.S. Senator.

Farrell is slender, medium height, with fair receding hair and a mustache (which he shaves off in later books). He has a black belt in karate. Despite his initial hostility, he becomes extremely close friends with Spenser and Susan. He takes turns protecting Susan periodically, as do most of Spenser's police acquaintances, but Farrell also helps out with tasks like rehabbing the house in Concord Walking Shadow and even walking/watching Pearl the Wonder Dog.

It's worth noting that Lee Farrell's appearance in the series is almost certainly related to RBP's coming to terms with his own son's homosexuality. While always relatively liberal, RBP nonetheless reacted somewhat badly to his son's announcement in 1990 (shortly before his scheduled wedding with a woman) that he didn't know if he could go through with the marriage because he was gay. According to a eulogy by David Parker, this led to a rift between David and his father. However, in later years they became closer again. Paper Doll was published in 1994 - just a few years after David's coming out. It's surely no coincidence that Parker's portrayals of gay people are more nuanced and less stereotypical in his later books.