Leo (played by Eric Da Re) is a trucker who also moonlights as Twin Peaks' primary source of narcotics, which he obtains from the Renault Brothers, and traffics it over the Canadian/US border for distribution in Twin Peaks. Because of his criminal versatility, Sheriff Truman has never been able to obtain any evidence of his criminal dealings.
Leo is married to Shelly, one of the waitresses at the town diner. Due to his volatile temper, he regularly beats her for both real and imagined transgressions, ranging from her infidelity with high school footballer Bobby Briggs to his dissatisfaction with the way she washes the kitchen floor. Shelly once said Leo married her with the intention of having a maid he wouldn't have to pay.
At the start of the series, Leo is one of the primary suspects in the murder of Laura Palmer. The plastic sheeting Laura's corpse was found wrapped in was identical to the sheeting Leo used as a temporary wall for his shack; his name triggered a positive response in a bizarre exercise utilized by Special Agent Cooper to determine suspects; and, most damningly of all, it was discovered that Leo was one of Laura's regular sexual partners, engaging her in escalatingly dark and violent sessions of BDSM. As it turns out, though, Leo was simply present on the night Laura was murdered by Killer Bob. Leo, Laura, Jacques Renault, and Laura's girlfriend Ronette Pulaski had been having an orgy when Bob came across Jacques' cabin; Bob waited for Jacques to go outside to urinate, and then attacked him. When Leo found Renault's unconscious and bloodied body, he panicked and fled the cabin, leaving Laura tied up and at Bob's mercy.
While under investigation for the Palmer murder, Leo is hired for his criminal skills by Ben Horne, who charges him with burning down the Packard Saw Mill with his business rival Catherine Martell inside so that Horne can simultaneously kill his competition and destroy the only obstacle to his obtaining the valuable Mill land. Leo decides to take advantage of the situation and use it to solve his own marital discord, abducting Shelly and tying her up inside the mill shortly before setting fire to it with a time bomb. Catherine foils his plans, however, freeing Shelly and helping her escape the mill.
When Leo returns home from setting fire to the mill, he discovers Bobby looking for Shelly. Leo attacks him with an axe, but is gunned down by Hank Jennings, hired by Ben Horne to cover his own tracks.
Leo spends most of the second season in a vegetative state, cared for by Shelly and Bobby, who agree to take him in as part of a scheme to commit insurance fraud. The plan backfires, however, when it turns out that Leo's home care is far more expensive than they had anticipated, and the pair are left destitute. To vent their frustration, they take to abusing Leo whilst simultaneously flaunting their relationship in front of him.
Near the end of the season, Leo regains some of his cognitive abilities and instinctively attacks Shelly and Bobby. Wounded during the course of a struggle, he staggers into the woods, where he is abducted by escaped mental patient (and former FBI agent) Windom Earle. Earle enslaves the barely-coherent Leo by affixing him with a shock collar and forcing him into submission. Leo spends the remainder of the series as a mute drone, serving Windom Earle.
In the third to last episode, Leo seems to regain some more functioning ability, as he frees another one of Earle's captives, Major Garland Briggs, believing Shelly's life to be in danger after Earle posts a photo of her on his cabin wall. In the series finale, Earle leaves Leo for dead, rigging a cage of poisonous tarantulas above Leo's head, with a string affixed between Leo's teeth; if Leo lets go of the string, the cage will drop on his face, releasing the agitated tarantulas. His fate is never revealed, although a shooting script indicated that he was supposed to be found by the Twin Peaks' Sheriff's Department, and that, upon seeing them break into Earle's cabin, he would have tried to speak, releasing the cage onto his face.