Officer Lou assists Marge in investigating the murders, showing up at the crime scene where Carl and Gaear have murdered the state trooper and the young couple who witnessed the crime. He brings Marge coffee to warm her up.
Lou isn't the brightest bulb on the force, and makes a pretty basic mistake in not realizing that a license plate starting with DLR is a dealer plate. Lou also helps track Gaear and Carl's trail to the Blue Ox Motel, and identifies the women they slept with.
He's a sweet guy, which is why Marge feels bad after she corrects him about the dealer plate thing. We're probably meant to think that he's the more typical small-town cop, compared to the smarter Chief Gundersen.