Sheriff Valenti is known for being hard on kids and teenagers. He's been the big bad wolf of Isabel Evans's nightmares since she was a little girl.
Description from The Outsider, chapter 2:
Everyone at Olsen High knew Kyle’s father. He did a locker search practically every week. He stopped anyone under eighteen who was driving even one mile over the speed limit. He showed up at practically every party, checking to see if there was any underage drinking going on.
Valenti is even cruel to his son Kyle. When Maria uses a stone to psychically watch Valenti, she sees him tearing into his son about doing the dishes.
Valenti is driven in finding aliens and he lacks general ethics while doing so. When he suspects that Liz Ortecho had an encounter with an alien, he takes her to see a dead body in a morgue to question her. When Alex Manes is stabbed and healed, Valenti is more interested in who healed him than who stabbed him. He shoots Nikolas Branson in the back because he suspects he's an alien, and he later shoots Ray Iburg. When Alex and Maria try to stop him from getting back to Project Clean Slate headquarters, Valenti repeatedly rams his car into theirs and pushes them over the edge into the arroyo.
For the 1999 Roswell TV series, the character is named Jim Valenti and he is initially the main antagonist of the series.
For the 2019 Roswell, New Mexico TV series, there are two Sheriff Valentis. Sheriff Jim Valenti died of a rapidly accelerated brain tumor before the start of the series. As in the books, Jim was Kyle's father and a member of the anti-alien group Project Shepherd. Unlike the books, Jim tried to help the kids in town and notably looked out for Alex Manes and Rosa Ortecho. Jim's widow and Kyle's mother became the new Sheriff Valenti after her husband's death. The main anti-alien antagonist of the series was Master Sergeant Jesse Manes, who was also a member of Project Shepherd.
↑Book 1, The Outsider, chapter 8: "Valenti reached up and slid off his sunglasses. He shouldn’t have bothered, Liz thought. His eyes were a cold gray that revealed nothing of what he was thinking or feeling."
↑Book 1, The Outsider, chapter 2: "And it wasn’t like he was some big, powerful-looking man. He was about average height, not much taller than Liz."
↑Book 1, The Outsider, chapter 2: "Isabel’s stomach clenched. Sheriff Valenti scared her. She did everything she could to avoid him. If he crashed a party, Isabel made sure she was out the back door. If he showed up at school, Isabel made sure she was quietly studying in a corner of the library"
↑Book 1, The Outsider, chapter 11: "When she was a little girl, she used to dream about him every night. Except in the dream he was a wolf, a wolf and Sheriff Valenti at the same time. In the dream he was always hunting her, sniffing and growling, and getting closer and closer to her hiding place."
↑Book 2, The Wild One, chapter 5: "When Isabel was a little girl, she used to have nightmares where Sheriff Valenti was a big wolf chasing her. He kept getting closer and closer, his long, sharp teeth dripping saliva. Even when she got older, she couldn’t shake the feeling that someday he was going to capture her, lock her away somewhere, and do all kinds of hideous tests on her—ending up with her on some dissecting table."
↑Book 2, The Wild One, chapter 13: "She should be wearing a red cape with a hood. She should be on her way to Grandma’s house, not in the mall. And the wolf shouldn’t have a gun. Why wasn’t the wolf following the story?"
↑Book 5, The Intruder, chapter 11: The fourth file had Sheriff Valenti’s birth certificate, the lease for the house, and a copy of his car registration. “Valenti’s middle name is Elmer. It’s just not as easy to be scared of someone named Elmer,” Maria said.