Lou Tyler (Kim Greist) was a villainess from "The Unluckiest Bachelor in L.A.," episode 7.18 of Diagnosis Murder (airdate February 24, 2000). She was a private investigator who was encountered by Lily Wilson, as Lily was looking for her brother.
It was revealed later on that Lou had known Lily's brother, Frederick Wilson, as she had been blackmailing him regarding his posing as a doctor. She colluded with Frederick in killing Lily, as she had falsely informed him that Lily was blackmailing him. After Lily's murder, the evil Lou faked Frederick's death by killing a man who matched his description; the fake Frederick's car went over a cliff. Lou was caught by Mark and Steve Sloan just as she and Frederick were on a plane about to leave the country, and both were arrested.
In Season Eight's "Deadly Mirage (Part 1)," it was revealed that Lou cut a deal in exchange for her testimony against Frederick. She was later shot and killed by Joan, Frederick's henchwoman, at the courthouse.