While there are certain basic characteristics shared by nearly all people, unique characteristics are most commonly found in the areas of physical appearance, personality, intellect, personal interests and hobbies. The combination of unique and common characteristics is what gives each person individuality.
LiveScience explains a psychological principle that defines the areas of personality which most strongly affect an individual's character, known as the Big Five. The Big Five personality traits include openness, conscientiousness, extraversion, agreeableness and neuroticism. Although these traits may seem basic, each one is heavily nuanced and can go a long way towards explaining the various unique characteristics of any given person.
Psychometric Success notes that a very early personality psychologist named Gordon Allport took a considerably different approach to studying what traits made humans unique. Allport divided personality characteristics into three subcategories, including cardinal traits, central traits and secondary traits. Cardinal traits are considered those so central to the personality that they are unlikely to change. Central traits are important, but are subject to change over the course of different life phases and experiences. Secondary traits are less important traits that may appear or disappear depending on circumstance. Personality psychologists have determined that truly unique traits are most likely to be found among cardinal traits.