Beast Wars' second season gives Dinobot a major role in two or three episodes leading up to "Code of Hero" where he's seen at his best, and then bites it at the end.
Seeing as season two of Beast Wars had 12 episodes, those three episodes do form 1/4 of the entire season, so it may not be a negligible amount of time.
The Simpsons used this trope in Season Six's episode "'Round Springfield". Bleeding Gums Murphy, the jazz musician who Lisa met in an early episode in the first season, faded into the background quickly (showing up here and there in crowd scenes for a while) and was almost forgotten until he appeared in the hospital in this episode. There, he tells Lisa his whole previously unrevealed back story, about how he was a successful jazz musician who made a guest appearance on The Cosby Show, and doing a saxophone duet with Lisa. Lisa goes off to school and wins a talent competition, and then returns to the hospital to be told Bleeding Gums has passed away. This occurs in the middle of the episode, and the rest of it revolves around Lisa's quest to arrange a tribute to him. After she succeeds, Bleeding Gums Murphy's ghost appears in the clouds in a parody of The Lion King (Mufasa, Darth Vader, and the CNN announcer briefly interrupt), and they have one last saxophone duet over the end credits before Bleeding Gums heads off for his afterlife date with Billie Holiday.
The Justice League episode "The Terror Beyond". Beforehand, Solomon Grundy was The Brute and Dumb Muscle with no motivation beyond greed. In this episode, Grundy's backstory is revealed, and he's given a very sympathetic motivation to fight alongside the good guys. Naturally, he dies fighting Icthultu and many tears are shed over him. One of his powers is the ability to come Back from the Dead, which was the whole reason he agreed to come along, as a Human Sacrifice was needed to defeat Ichthultu and someone like Grundy was going to present less of a moral dilemma to that end. He returns for an episode of Justice League Unlimited but has Came Back Wrong and devolved into The Berserker and has to be killed by Anti-Magic, again Played for Drama. Given the manner of his death, its not clear if he will again return from the grave or was finally Killed Off for Real, but regardless he does not reappear in the series again.
Nabu was introduced in Season 3 of Winx Club as Aisha's love interest ...and that's all. However, the final episodes of season 4 focused on him a lot, showing his sheer badassery, to the point of beating one of the four Big Bads all by himself. Shortly after that, he performed an Heroic Sacrifice.
In South Park, Kenny, although always part of the main cast, barely had any effect on the plot up until "Kenny Dies", after which he is killed off (for a season).