William is Victor’s younger brother, and the guy who taunts the monster for being ugly when he encounters him in the woods. He tells the monster he is a Frankenstein, and the monster kills him for it.
Oh, William – you’re not the brightest crayon in the box. Nor are you the most considerate, the kindest, or the most interesting. You’re pretty much the embodiment of all the shallow negativity of the other characters in the story. And, fittingly, you’re the monster’s first victim.