Helena's Links
Aunt on his mother's side.
Took advantage of his arrogance and selfishness to transform him into a weapon she could use to take power for herself. It backfired in the end, when she could no longer control him anymore.
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.
When their families first made connections she may have had some fleeting feelings of romance for Theradon but those quickly turned into lust for the throne beside him; she desired to become his queen and take the full throne for herself. But Theradon did not show any interest in her, he rejected her advances and instead married Helena's sister.
After this Helena loathed Theradon and her lust turned into jealous rage as she plotted to steal the throne out from under him through his firstborn son, Victor.