Mosp from Sluggy Freelance died almost immediately after she was given her own week-long arc detailing her backstory.
A brief story arc in Something*Positive focused on Faye and Fred, with Fred planning to tell Faye about his Alzheimer's diagnosis at the end of a day they spent together but losing his nerve. The next morning, he woke up - and Faye did not.
Homestuck did this for several deaths in a row, giving characters lots of screentime in a flash page right before killing them. First "Kanaya: Return to the core" gave Eridan and Feferi a lot more screentime than they're used to and dove into developing Eridan's character for the first time ever, just before Eridan murdered Feferi, and got killed himself soon after. Not much later, "Equius: Seek the highb100d." was nothing but Equius and Nepeta getting the most screentime and character development either had ever had, and was shortly followed by both of their deaths.
It also did this to genuine-major-character Kanaya (and to a lesser extent Tavros) around the same time, though only one of those stuck, and to Rose's waking-self. In fact it's safer just to say that Andrew Hussie likes this trope in general.
Newbie hunter Victoria in Charby the Vampirate was shown interacting with a fair portion of the cast and given a distinct personality, motivation and drive right before being unceremoniously murdered by Quixoto while he used his mind control to prevent Charby from interfering.