My story happens as a result of one of the main characters getting murdered. I'm wondering how to get readers to care about someone who's not even in the story (as far as the readers know anyway) or at least sympathize with the other main character and their relationship other than "She was probably his girlfriend or something".
Her (their) backstory is not that interesting without the context of the current, post-death storyline so I don't really want to start with 5 chapters of backstory that will be boring for the reader until chapter 10.
I have an idea about doing some kind of flashbacks, something I later realised wuold be quite similar to what the "Arrow" TV series does, but I am unsure how to write this effectively without confusing the reader.
How can I write these flashbacks in a reader-friendly way? Or if anyone have some completely different idea of how to write something like this.
While several of the answers gives an interesting approach about basically not telling the backstory it is not quite what I am looking for so I thought I should give more information.
The backstory is not just about her. She and the main character have done a lot of things that are still affecting the future either directly or indirectly.
The main story will take place several years after the murder so he has gotten over the initial shock (i.e. he is not lying awake at night crying about her) but he still has to deal with the consequences of their time together.
An example would be that the main character might replace her with a android that they built together (not quite what will happen but easier to explain) and I want to both tell the background story of how they built that android as well as how said android differs from the real version (speech, personality etc.).