Who is Olly from The Acid House and what is their importance?

Olly (Olivia) is Brian's girlfriend. Their affair is blackly comic, in that they appear to be using one another for sex, and yet neither of them feels used, because this is the norm for both of them. At one point, Brian reflects to himself that he prefers to make love with someone he cares about, rather than merely having sex, but he also seems quite comfortable with the casual arrangement. After their break up, the two encounter each other socially and are friendly, but neither one pines for the other. Olly is described as envious.