William is the youngest son of the Frankenstein family.
William Frankenstein is a Character that is only spoken of in the book about two or three times. Much of what you learn of William is in chapter 6-7 in chapter 6 when victor gets a letter from Elizabeth; she goes on to talk about her darling cousin William explaining that he has “he is very tall of his age, with sweet laughing blue eyes, dark eyelashes, and curling hair.”(6.51.2) and that is all we hear about young little William Frankenstein.
Then in chapter 7 victors now gets a letter from his father, explaining that young William was killed, by who they don’t know at this time. Young William was found by his father the next day after William had gone missing the night before. “About five in the morning I discovered my lovely boy, whom the night before I had seen blooming and active in health, stretched on the grass livid and motionless: the print of the murderer's finger was on his neck.” (7.59.2)
The last time you hear about William is when the monster is telling the story of how he meet and kills the boy.
“Urged by this impulse, I seized on the boy as he passed and drew him towards me. As soon as he beheld my form, he placed his hands before his eyes and uttered a shrill scream: I drew his hand forcibly from his face, and said, `Child, what is the meaning of this? I do not intend to hurt you; listen to me.” "He struggled violently. `Let me go,' he cried; `monster! ugly wretch! you wish to eat me, and tear me to pieces--You are an ogre--Let me go, or I will tell my papa.'
"`Boy, you will never see your father again; you must come with me.'
"
`Hideous monster! Let me go. My papa is a Syndic--he is M. Frankenstein--he will punish you. You dare not keep me.'
"`Frankenstein! You belong then to my enemy--to him towards whom I have sworn eternal revenge; you shall be my first victim.'
After hearing that Williams last name was Frankenstein, The monster go’s into a massive rage and then kills young William.
http://www.literature.org/authors/shelley-mary/frankenstein/chapter-16.html
"The child still struggled, and loaded me with epithets which carried despair to my heart; I grasped his throat to silence him, and in a moment he lay dead at my feet.
"I gazed on my victim, and my heart swelled with exultation and hellish triumph: clapping my hands, I exclaimed, `I, too, can create desolation; my enemy is not invulnerable; this death will carry despair to him, and a thousand other miseries shall torment and destroy him.”. William death was at the hands of the monster renders him a symbol of lost and violated innocence.