Who is Peter Sissen from The Virgin Suicides and what is their importance?

Peter Sissen was the first of the neighborhood boys to set foot in the Lisbons' house. He had helped Mr. Lisbon set up a mechanical model of the solar system in his classroom. As a reward, he was invited to the Lisbons' house for a dinner with the family. During the meal, he excused himself to the bathroom and entered the one located upstairs. He then searched the area for secrets and later reports them emphatically to the rest of the boys.