Who is the character Assef in chapter 5 of "The Kite Runner"?

In chapter five, Amir and Hassan walk to the pomegranate tree and travel through a barren patch of rough land, where they are stopped by Assef and his two friends Wali and Kamal. Amir describes Assef as a tall, blonde hair, blue-eyed teenager, whose father is an Afghan and mother is German. Assef is known throughout the district for his cruelty, and Amir mentions that he is a sociopath. Assef is immediately introduced as an antagonist in chapter five as he threatens Amir and ridicules Hassan. Assef has a reputation as a violent individual and even carries brass knuckles with him at all times. Assef also earned the nickname "the Ear Eater" after he bit off a boy's ear in a fight over a kite. He reveals his wicked personality by commenting on his admiration for Hitler and desire to ethnically cleanse the Hazaras from Afghanistan. Before Assef can harm Amir, Hassan comes to his rescue and threatens to knock his eye out using his slingshot.

Can you describe Assef's character as a teenager in The Kite Runner?

It's hard to explain why anyone becomes a bully, but Assef seemed to enjoy this role as a teenager. He took bullying a step further when he gleefully sodomized Hassan, claiming

"... there's nothing sinful about teaching a lesson to a disrespectful donkey."  (Chapter 7)

To Assef, the Hazara boy Hassan was little more than an animal and, as an adult, Assef makes an easy transition from homosexual rape to ethnic cleansing. Assef's perverse actions are based on the hero worship he felt for Adolf Hitler, and as a Taliban official, he faithfully followed in Hitler's genocidal footsteps. Assef was born into a wealthy and privileged family: His father, an airline pilot, was a Pashtun, the ethnic Afghan people who historically hated the Hazaras. Assef's mother was from Germany, and he inherited her love of Hitler as well as her blonde hair. Assef preyed on younger boys like Amir and Hassan, always accompanied by a small gang of enthusiastic followers. Amir's shame of Hassan's ethnic background (and his own cowardice) partly contributed to him never telling Baba about Assef's bullying, so Assef continued his work with impunity. He believed, like his father, that Aghanistan should be "purified" of the Hazara, and after the Russian occupation of his country, he became a Taliban soldier, where he could

"... take out the [Hazara] garbage."  (Chapter 22)