I know there are far better examples than this but this topic came into my head when I was randomly thinking about Sonic Adventure 2. It was a pretty good game when as I remember and had a pretty interesting story. During the end Shadow, introduced in that game, and Sonic save the world by doing a special ability but in the process Shadow apparently dies. The characters then reminise, badly I admit, about Shadow's purpose in life among other things. Well as we know Sega not one to ruin a chance to exploit new Sonic characters made a bad game featuring him and also included him in a lot of bad games with Sonic in it.
Any media will do for this topic. It doesn't have to be a video game.