Queen Annabeth's Links
She is Annabeth's older sister. She was always envious and jealous of the attention and praise her younger sister got. She loathed her sister's kindness, viewing it as weakness. When Theradon rejected Helena's advances (as she was seeking power not love) and instead Annabeth was wedded to the king, becoming queen, Helena's fury was ingited.
In the end Helena ended up killing her sister through nefarious means as part of a grander plot to take the throne. Annabeth was the only dragon Helena ever killed with her own talons and not through a puppet.