Amir and Hammad stand as opposites to each other in many senses, which conveys to the reader that not every member of the terrorist organization is the same. They are individuals rather than a grouped "Other." Starting with physical characteristics, Amir is described as a "small thin wiry man" in contrast to Hammad, who is "a bulky man, clumsy."
In addition to describing differences in the two men's physical characteristics, DeLillo utilizes the character of Hammad to add a sense of humanization to an evil entity. While Amir leads the attack and preaches self-sacrifice for their beliefs, Hammad does not strictly adhere to their system and often exhibits self-centered, sacrilegious behavior. For example, he masturbates, engages with women, and only prays some days when he sees fit. At one point he even fantasizes about getting out of his car and into a car with a group of laughing American young people, leaving his current life behind and pursuing a different path.
Amir is essentially the poster boy for perfect behavior and beliefs, with an unwavering faith and a deep desire to complete the attack. Hammad is self-seeking and not dedicated to the cause. Even in his final moments on the plane, he fastens his seatbelt, prioritizing his safety over the mission.