I would like to get some opinions out there. I was on the fence on how I would like to develop a sub-character in a short story romance/erotica piece as I don't want to take away too much from the good parts. This sub-character feels necessary and helps to support the story of how the main got to where she was. For that reason, I have decided to develop him, even though it adds quite a bit more text than I was planning. As I did, I realized that I was actually jointly developing the sub-character and the main character's back stories, which led to the main characters current state of mind and motives. Does the forum think this might be ripping off the sub-character's own back story?
It's good when you can show how the characters are connected both through back story and the main story. It sounds like you're on the right track.