A good way to write a character sketch is to tell a little story about one encounter you had with him or her. If you do that, you could describe a place briefly, hopefully a place that belongs to the person you are describing, focusing on things in the scene that are somehow representative of the person you are describing. Describe how the person is dressed. Then simply tell what happened as you spent time together. From time to time, describe the person's gestures or facial expressions. It is important to put words into the person's mouth in direct quotations.
As you work on this paper, you should decide what kind of emotional reaction you want the reader to have in relationship to this person. What kind of details can you select to create that emotional reaction? Avoid making broad characterizing statements; instead, let the details you give suggest general characteristics. Let the reader draw her own conclusions.