Define what makes your character relatable, what helps him stand apart from the crowd. Relatable features might be where the character lives, where he works or some background on his family. The items that make him stand out might be that he has psychic ability or is president of an organization. This information can help define the character.
Provide some history of your character that might explain behavior or perspective. Even if you do not believe that nature shapes perspective, some will see the environment the character lives in to have some role on his behavior and world view. Including some of this information can be very beneficial to the reader and for guiding your writing.
Offer some physical characteristics that will help the reader envision the character, even without a visual accompaniment. What you choose to include will not only help you envision the character as you are writing the piece, it can help shape the mental image the reader conjures when reading it.
Include anecdotes that show how your character behaves. Don't tell your readers how he behaves. Telling the reader that someone is generous is not as effective as an example of where they selflessly gave to someone. Anecdotes also allow the reader to assess motive, which also helps define the character.
List a few favorite things that will help the reader better understand the character. If the character has a favorite type of music or a favorite hobby, such as jazz or painting miniatures, these can be pivotal in helping to define him.
Explore the goals of your character. What someone wants out of life can often say a lot about who the person is. For your character personality profile, describe his hopes and dreams. Similarly, you can indicate that he does not have any goals, which also helps define his personality.
Look to personality profiles as inspiration for your own character sketches. For example, the Myers-Briggs Type Indicator defines different personality types. Using one of these personality types, such the guardian or the nurturer, can help make your character more easily relatable.