According to the Purdue Owl, a good descriptive essay includes clear and powerful language, sensory description, tone and agreement. Since an essay describing a person is a form of descriptive essay, this is a good formula to follow.
When describing a person in an essay, it is important to focus on the senses: sight, sound, taste, smell and feeling. A good description appeals to as many of the senses as possible. It is not just important for a reader to understand what the person looks like, but how the person sounds, smells and makes people feel. Essentially, readers should feel as if they know the person intimately after reading the description.
Word choice is a key element in creating the right image. Vague words don't provide much individualization when describing a person. Conveying a person's personality is sometimes one of the most challenging aspects of writing a descriptive essay. In addition to descriptive words about behavior, this can be accomplished in a person's physical description. Someone who is cold might have "steely eyes" and "pursing lips," for example. The description should also have a consistent tone that is in agreement with the image of the person that is being conveyed. A description of a happy person with sad features, for example, leaves readers confused and without a clear image of the individual.