Every character needs a compelling back-story. But what happens if your side-characters insist on having their back-story told, in detail? Well, as an author, at first you're very happy, thinking you have a short story on your hands, something that can be used as a bite-sized intro to your series. But when that 'short story' keeps growing, complete with new characters and poems and songs, and still you're not done, then you have to graciously accept the fact that you have a spin-off series on your hands.
This happened with Heinregard and Clayten, two of my side-characters. Their back-story has spawned an entire novella series that I'm calling the Seasons Cycle. The first novella, Spring Song, tells the story of when they first met. It also helps establish more background and history for my main series, as the spin-off is set twelve years earlier.
If writers knew everything about their characters from the get-go, there would be no surprises. As our characters go through different situations, their reaction and response give us insights into their personalities and motivations. Clay is a 'dark horse', a seemingly minor side-character who turned out to be a pivot and anchor in my main story. Finding out more about him and Heinregard as I write the Seasons Cycle has been an indulgence, a true pleasure. Their songs are some of the most beautiful I have ever written, and I hope my readers and listeners enjoy listening to them as much as I enjoyed composing them and bringing them to life with the help of a team of talented musicians.
So yes, even side-characters should have compelling back stories, because every character is a main character in their own life/world. And when they insist on having their back-stories told, listen up - the Muses call for a reason. In a world that has regressed, when darkness threatens on all levels, it has become even more important to stand up against bias and prejudice, hatred and discrimination. In a genre that has become increasingly dark and violent, I have held to Light and Love. When romance fiction is now rated at steamy/flame levels, I have kept to slow-burn and sweet. If that appeals to you, be sure to check out my novels and novellas, where the writing is lush and lyrical, and the longing is real.