Who is Rachel from About a Boy and what is their importance?

Will meets Rachel at a party. She is described as beautiful, single, and exciting. Sadly, the only thing that will can think of to talk to talk to her about is Marcus, which leads Rachel to believe that Marcus is Will's son. Will doesn't correct this assumption until he realizes how much Rachel means to him and tells her the truth. This does not lead to a break up as Will might have feared, but to a more honest relationship that later blooms into exactly the kind of relationship Will has always feared.