The Skullboy :

... According to a Legend, it tells that a mysterious artifact known as the “Skull Heart” has the power to grant a young man’s wishes. Of course, there’s a catch. If he has an impure heart, even a selfless wish will be twisted in the most vile of ways : peace brought through brutality, eternal life through perpetual childhood, or a lost love returned as an undead monster. Hundreds have sought the Skull Heart, but so far none have been deemed worthy and spared its cruel power. Those judged impure are transformed into the fearful instrument of their twisted wish.

Filius was once just an average schoolboy, but one day woke up without any memories and a second mouth on the back of his head. He was now playing host to a Parasite named Samson, an unruly mop of demonic hair with incredible power. With no memories or anyone to turn to for help, if he ever hopes to piece together his past and survive the inevitable clash with the Skullboy, Filius will need to trust this mysterious being...

To get his memory back, he decides to search for the Skull Heart and make a wish. At lunch time, he eats in a corner to not reveal his parasite Samson but Samson does whatever he wants, without listening to Filius. While Samson grabs a hamburger, Filius sighs because he doesn't have any control over his hair... :

Filius : "Come on Samson, it's because of you that I'm getting overweight..."

Samson : "I have the right to eat what I want! Kiddo, think twice with your milkshake."

Then you just pass by and you see your classmate with a parasite on his hair... ... ...

Filius : "Hey, don't be scared! We don't mean any harm!"

Samson : "Hahaha! This human is a real chicken!"