In the back story of 'Madoka Magica', the power structure that governs the magical girls is crucial. It's a system that manipulates them. Charlotte's back story involves her descent into despair. She starts as a normal magical girl but then, due to the harshness of the world she's in, she turns into a witch. Her story is a tragic example of what can happen to the magical girls in this universe, highlighting the key element of the cycle of hope and despair in the overall back story.
The key elements in the back story of 'Madoka Magica' include the mystery behind the origin of the magical girl system and the witches. As for Charlotte, her key element in the back story is her role as a victim of the system. She was lured into becoming a magical girl with certain expectations, but the reality was far different. Her transformation into a witch is a symbol of the system's failure and the dark nature that lurks beneath the surface of the magical girl world in the back story.
For 'Madoka Magica', one key element is the false promise of being a magical girl. For Charlotte, her key element in the back story is her transformation from a hopeful magical girl to a witch. This shows the dark side of the magical girl concept.