After her father was excommunicated from the church for heresy, Kyoko's family starved and only had enough food to survive.  She formed a pact with Kyubey for people to believe in his sermons again in exchange for becoming a magical girl.  But eventually her father discovered the truth and condemned her as a witch.  He burned himself to death along with her mother and sister, leaving her all alone.  As a result, she wanders aimlessly in search of witches to hunt.  She's now obese, 800 pounds. 