Brad is a witch hunter
A Witch Hunter is a mortal who is born with a witch hunting gene. Witch hunters aren't usualy aware of their abilities, but they have the power to sense magic, which often makes them suspicious of witches and creates an urge for them to uncover the truth. If a witch hunter announces that someone is a witch, the witch turns into a mouse for one hundred years. A witch hunting gene (which looks like a pair of tiny blue jeans) can be removed by witches, although it is a very delicate and dangerous procedure. If the gene is destroyed, all witches transformed by the witch hunter will be changed back to their human form.
A witch hunting gene is affected by the Earth's atmosphere; so in certain weather conditions, a witch hunter's powers may be dormant.