Supportive people, are steady and serene. They are sympathetic to others, and like the name suggests, eager to support the important people in their life.
They are group-oriented people who put the team first, often ahead of their own interests. They are good listeners that often prefer to be in the background of large groups, but when they do speak, they like to be given time to articulate their views without interruption. They tend to be very polite and considerate of others, and so might hesitate to say something that could offend or raise unnecessary conflict.
Because they will not easily challenge you, to understand what is really going on in their heads, you might need prod and ask many "how" or "why" questions to get at the root of their feelings.