Capt. Phillipa Georgiou - Star Trek: Discovery
Captain Georgiou was instantly memorable, and her death early on in the series was huge loss in-universe and out. The continued presence of mirror!Georgiou is a constant reminder of how we're stuck watching a xenophobic, former dictator instead of one of the best officers in Starfleet to ever captain a starship.
Nolan Hotchkiss - PLL: The Perfectionists
Most of PLL: The Perfectionists' premiere was devoted to showing us Nolan's dickish behavior. Then we discovered he knew his sister faked her death, was helping her take down Hotchkiss Technologies, and regretted having to treat the people around him horribly. Just as we started to root for him, he was murdered.
Det. Eudora "Dora" Patch - The Umbrella Academy
Diego and Dora had a classic, "he's a vigilante; she's a detective; they're on and off romantically dynamic." It came to an abrupt end after Dora was murdered four episodes into the series.
Jesse McNally - Buffy the Vampire Slayer
Remember, Jesse, he was Willow and Xander's best friend. Not long after Buffy moved to Sunnydale, he was kidnapped and turned into a vampire. He could have made for an interesting recurring villain, but alas, Xander accidentally staked him.
Georgia "George" Lass - Dead Like Me
At 18, George was a depressed college dropout living with her dysfunctional family and forced into working a menial temp job. Her life was brimming with potential TV storylines. We never got to see those stories play out because a toilet seat from outer space crash-landed on top of her.
Victor Stecker-Epps - Burn Notice
Like Michael, Victor was a burned spy coerced into working for a rogue organization. Initially antagonistic, he eventually agrees to team up with Michael to take down their common enemies. Unfortunately, their partnership began and ended in the same episode.