I'm too lazy to design her soul gem and yet I'm not lazy enough to not make a depressing background:
A few months ago, Kusa's brother and mother got into a nasty car crash. Her mother was hospitalized and her brother died. Kusa incredibly upset, so much so that Kyubey appeared to her and she wished her brother hadn't died. Her wish came true and Hampton "came back to life" (however the doctors said, to make sense of the situation, that he was in some sort of coma). Thanks to Kyubey, Hampton also had no serious injuries and got out of the hospital in a week. Kusa, elated to have her brother back, spent most of her time with him. However, Hampton caught her one night when she was trying to sneak out of the house to kill witches. He begged to go with her and she reluctantly agreed (he didn't know he was going to watch her fight witches, though). When in the labrynth, Hampton was petrified, begging to get out, despite not being able to. Kusa was able to avoid most of the minions as the two made their way to the witch, though a few minions attacked her and Hampton, which injured him a little. Though when the two got to the witch, it didn't take too long before Hampton was almost killed, just barely being saved in time by Kusa. Kusa killed the witch, fueled by her rage When she and Hampton got out of the labrynth, Hampton was disgusted, calling her a monster and wishing he wasn't the brother of such a creepy and dangerous girl, which broke Kusa. Hampton now ignores and shuns Kusa, while Kusa herself becomes more corrupt each day, only slowing the process through the grease seeds. She's lost quite a bit of her sanity and mental stability and now takes great joy in brutally murdering the witches, as if it were a way of patching up the hole in her heart torn by Hampton.