|Family|| Gordon Сlaw (grandfather)|
|Location||Some sort of boarding school|
|Portrayed By||Alex Doduk|
William "Billy" Thaw is Dr. Claw's nephew, Talon's cousin and one of Penny's romantic interests. He only appeared in "Claw's Nephew."
He is sent by his grandmother to stay with his uncle, Dr. Claw to become evil just like all of his relatives while he is on spring break from boarding school. He meets Penny, who is on a mission with Inspector Gadget and the Gadgetinis. Penny quickly develops a crush on him and he takes her and the rest of the Gadgets to his uncle's house for dinner. Gadget, being the bumbling idiot he is, is oblivious to the fact he was in Claw's lair. When William found out his uncle was evil, he called his grandma to warn her. Of course, his grandma was evil too and didn't understand at all. This caused Claw to blow up his lair and for William and the others to escape safely. However, Claw and MAD Cat escaped as well in Claw's plane. It is never revealed what happened to William at the end, but he probably went back to boarding school or with his grandmother.
- In the 2015 series, Claw has another nephew named Talon, who is also William's cousin. Penny also has a crush on Talon.