Leo Balmudo, also called Crater-Face, is the pock-mark faced leader of the Scorpions, a gang of bullies and the greaser rivals of the T-birds. Though not a Rydell student, it's more likely he's a drop-out, possibly from St. Bernadette. Although he is the main antagonist, his role in the film is small.
Leo is cruel, and a world-class jack-ass. When he and his girlfriend Cha-Cha visited the local make-out area, he backed his car, Hell's Chariot, into Kenickie's 1946 Ford, interrupting Kenickie and Rizzo's make-out session. When Kenickie called him "crater-face", Leo backed his car into Kenickie's again, knocking off the fender. When Kenickie told Leo that he was gonna pay for that, Leo responded evilly, remarking that the car was worth only 75 cents, including Rizzo.
Later, Leo and the Scorpions were seen nearly running some students over near the Frosty Palace.
Prior to the National Dance-Off, when Rizzo (who'd previously broken up with Kenickie) and Marty Maraschino were bemoaning their lack of dates, Leo happened by and offered them a ride, eventually becoming Rizzo's date to the event.
At the dance-off, Leo became enraged at Kenickie for making moves on Cha-Cha, perhaps she and Leo also had a falling-out, and eventually, during Born to Hand Jive, he and Kenickie brawled.
When Kenickie was accidentally knocked out when Putzie opened the door and hit him in the head, Danny Zuko took over the reigns as Greased Lightning's driver. Leo pulled a few damaging tricks, gouging Greased Lightning's door and knocking off a hubcap with his car's bladed hubcap. Despite that, Danny jumped Greased Lightning over a hurdle, and won the race, while Leo's car stalled in Thunder Road's deep puddle.
Leo lost his car to Danny, who apparently gave it to Kenickie, eventually turning Greased Lightning into a red mad machine.
Though having a pockmarked, craggy complexion, Leo wasn't necessarily ugly, but he was mean-looking, and lived up to that.
He first confronted the new T-Birds on the Rydell Track. It wasn't a violent moment, but could have escalated, especially after Louis DiMucci talked tough "We should take care of them tonight." After Johnny avoided a brawl by stating that they were bowling that night, Davey Jaworski goaded, "Hey, you're lucky, we're bowling tonight!" causing Leo to admire Davey's brashness, "I like that."
A few months later, back at the Bowl-A-Rama, Leo showed up alone, and was bravely insulted by Dolores Rebchuck as she skated by. Thinking he was alone, the T-Birds rushed to confront him... and were met by the entire gang, who'd showed up a mere minute before the T-Birds. Balmudo made a snake-hissing sound, and scared off the T-Birds. Davey, however, was still inside, unaware until Dolores told him, and he rushed to assist. Davey barged out, demanding to know where was that "Rat-faced Balmu...?!" Leo grabbed him, was abut to tear Davey apart, and was swiftly kicked aside by a hero, Lone Rider. Leo and the Cycle Lords were soundly stomped, and they retreated when they heard the approaching police sirens.
At the Lani Kai Lani Luau, Leo and the Cycle Lords crashed the party. However, asses would soon be kicked when Lone Rider (Michael) emerged, alive and well. Needless to say, after Lone Rider soundly trashed the Cycle Lords – again - Balmudo got his comeuppance, and was knocked into the pool by two new Rydell heroes, Stacie & Gracie. It can be assumed that he and the Cycle Lords were arrested afterwards.
Though pretty mean to his rivals – and at times, people crossing the street, as seen in Grease - Leo wasn’t wholly bad. He generally was decent to the manager of Pfeil's Cycle Salvage, G.E. Salva, who usually got him used motorcycle parts at a substantial mark-down. Perhaps Balmudo could have avoided jail time, and entered a plea bargain; Work for Mr. Salva, rebuilding bikes for customers. Just a thought.