What is Heather's last name in the book "Speak"?
The character of Heather is introduced in the book as a friend and "foil" to Melinda...meaning Heather brings out the best and worst qualities in Melinda and is there to help Melinda come to terms with events in her life.
I have looked in my personal copy of this book as well as on several websites and can not locate the last name of this character. I am prepared to go out on a limb and say that this character has not been assigned a last name.
This is probably because Melinda "used to be like Heather" and is upset that events in her life have made her unlike Heather. Melinda has nicknames or refuses to use the names of people whom she does not respect or like. I wouldn't go so far as to say she doesn't like Heather since Heather is one of her only friends after the rape, but Heather represents to Melinda an aspect of Melinda's happier, former self. Perhaps Heather doesn't have a last name intentionally to protect or shield Melinda who is resentful that she is a victim and has been robbed of her bubbly and carefree life.