Rex Buckland was the main antagonist for the start of Charmed. He and Hannah Webster were disguised as humans, when they were really warlocks, everyday plotting to steal The Charmed One's powers. After a few trials, he is sadly killed indirectly by The Charmed Ones.
History
The real Rex Buckland founded the successful of the eponymous Buckland's Auction House, which specialised in rare and valuable antiques. Shortly before the Charmed Ones gained their powers, a couple of warlocks killed Buckland and his coworker Hannah Webster, took their identities and intergrated themselves into the Buckland's workplace seamlessly.
Meeting Prue Halliwell
After Prue quit her job at the American Museum of National History due to a falling out with her boss, she applies for a job at Bucklands in the episode I've Got You Under My Skin. Rex agrees to an interview with her, but despite her running late she bumped into him while in the elevator, where they have the interview afterwards. After succeeding, Prue has a second interview where Rex and Hannah test her abilities with various objects. Hannah accidentally knocks over a open paint can, causing Prue to flinch and use her telekinesis to avoid being hit by the paint. They notice, with Hannah believing that she is a witch.
Death
In the episode Wicca Envy, his life comes to an end. The Halliwell's are about to hand over their powers when Leo intervenes and they are returned. Hannah then shapeshifts to a black panther and leaps out to The Charmed One's. By reflex, Piper freezes them, not knowing her powers returned. They then move Rex in front of Hannah as a panther and unfreeze them. Hannah then lands on Rex, killing him, and shapeshifts back where she pleads for mercy when they both fail and return to the Underworld