How developed should a character be who only shows up in flashbacks and not in the main storyline? I've got this character who's got this brother. My character remembers going to a restaurant the last time he saw his brother. The story is about something else, but this character also has to take care of something for the brother. Other than that, the brother is not at all important to the story. So, how important or relevant is it to put too much into this brother? I mean, I think I've got it, but it's pretty bare bones.
I don't know that you'll be able to give him that much dimension in one single scene. But you could maybe flesh it out a little bit in the MCs thoughts and memories? The brother may not be important, but giving the relationship between the two more depth would also give it to your MC.
Doesn't seem at all important to me. Usually, my advice would be to cut him entirely if he's not needed.
If the brother won't appear much in the story, I think there's no point in developing him too much. If he's not going to be mentioned very often, you might want to give him a memorable name that the reader can easily link back to him.
How much does the brother mean to the character? I agree with all of what has been said above...but if he's actually very important to an important character, he is important. Thus, he needs to be well-thought out and round instead of flat. Even if you have little opportunity to reveal much about him, that's okay - we don't need to know his favourite colour or last girlfriend (unless, of course, that is essential to the plot) but he still needs to be an interesting person on the page when we do occasionally meet him.
One thing I like to do with my minor characters is to get to "know" them better outside the story. I usually develop a list of their likes and dislikes, what they do for a job, what their main goals in life are, what their relationship with family members is like, and so on. I might even write a brief scene or two from their life, or rewrite a scene from their point of view. That stuff may never make it into what I'm working on, but sometimes just my viewing them as a real person rather than just someone who serves a purpose for my MC helps them to come across more realistic in subtle ways.
