Charlotte's story is of a girl with a terminal illness (and or) had a mother with a terminal illness. Kyuubey Approches Nagisa and offers to grant her any wish in the world. She initially thought to cure her mother's illness, but thought a wish like that would be too great and settled for a single slice of cheese cake, to share with her mother one last time.
After her mother passed away she was then questioned by kyuubey "Why didn't you just simply wish your mother's illness away? It would have been easy for you." With the despair knowing she could have full well saved her mother and knew she could but doubted herself, she fell into despair and became a witch…
The witch of sweets, Charlotte.
-This is an official backstory for Charlotte that was noted in the production notes for the TV series. It is also made note of the information that I've previously mentioned, a one-of-a-kind Witch whom lets people walk in freely and does not bother to lure them in with kisses and curses. She is a lonely witch whom only desires the company of others, but if she is startled or attacked, she will devour those who trespass against her.