Alexa Swan, the Demi-God.

18 y.o Alexa "Lexa,lex" swan

Alexa is the daughter of Athena and Poseidon but thinking she would be better off with parents the didn't hate each other, they bound her 'goddess' form and placed the unborn baby into a human woman's womb. The child was born On may 20th.
Renee Swan had two children to Charlie, Alexa and Isabella, never knowing about her first Borns true parentage.
When she left Charlie she took both girls with her. On Alexa's 13th birthday Chiron came to Renee telling her about a camp for talented and gifted children. That summer Alexa went to camp half blood, and a year after that Renee was told that she was accepted into a boarding school for gifted children.
From then on Alexa stayed at camp during the school year and, per her mothers request, spent every other summer at home with her mom and sister.
Of course the sisters stayed in touch, and as they grew older Alexa told her little sister about camp half blood and her real parents and why she had to stay away as long as she did.