Andy is a chilling villain. He's a high school senior who seems to have a long history of sexually assaulting girls. If the response to Melinda's anti-Andy graffiti is proof, Melinda is not the first girl raped by Andy, nor is she the first to keep quiet about the attack.
Andy is basically a flat character. We see him as a grim, shadowy monster scaring the heck out of Melinda. We know his family is wealthy, that he's good looking, and that he's very charming. We don't know why he does what he does.
Andy is important to the novel as an antagonist, but the focus of Speak is Melinda, like the focus of Alice Sebold's The Lovely Bones is Susie Salmon, another teenage victim of rape. In Speak, as well as in The Lovely Bones, the experience of the victim is more important than trying to understand the motives of the attacker.