Arex Core was the son of rogue force-wielder Tred Core, a man who terrorised the Galaxy during the Clone Wars until he was killed by Harash Soru.
Arex was discovered shortly after the end of the Galactic Civil War and trained to be a Jedi, Hara keeping the secret of his father hidden from Arex. However, on a mission to Tatooine, Arex met with ROvaniss who told him of his Father. Enraged, Arex fell to the Dark Side and swore vengence on his father.
During his time as a Sith, Arex became close to Darth Gerett due to their desire to see Soru dead. However, Arex did not realise that Gerett was merely using him to get to Soru.