One of the most heartbreaking deaths on Lost was the death of Dr. Juliet Burke (Elizabeth Mitchell). Juliet was introduced in the third season of the show as one of The Others, working against the plane survivors. As the series progressed, Juliet formed some close bonds with some of the survivors and even a relationship with Jack and then Sawyer. This made her more popular with audiences, soon becoming a fan-favorite character. Unfortunately, the character met her demise in the first episode of the shows sixth and final season, paving the way for the conclusion of the whole series.

https://screenrant.com/lost-juliet-death-flash-sideways-timeline-theory-explained/