I'm going to assume from your question that you have not yet read the novel. I want to give you a glimpse of the main characters but not tell too much, as it would spoil one of the best parts of the plot. I highly encourage you to read this book. It is definitely worth your time.
The protagonists of the novel are Jane Jerome and Buddy Walker. Both are 16 years old. At the very beginning of the story, Buddy is involved with a group of teens who vandalize Jane's house and harm her sister (the premise of the story). Ironically, the two end up in a romantic relationship which of course, excludes this major piece of information.
Harry Flowers is the ringleader of the group of boys who trash Jane's home. He is rich, spoiled, smart, and bored. These qualities combined most often with alcohol create nothing but trouble.
The Avenger is the neighbor of the Jerome's who witnesses the vandalization. His identity is left somewhat of a mystery, but his role in the story is suggested by his surname. He plans to personally avenge the four boys involved in the crime at the Jerome's house.