The identical twin brother of Acheron Parthenopaeus, Styxx was born to wealth and power. During their mortal lives, Styxx believed himself to be the older brother. Because of Acheron’s life choices and refusal to listen to Styxx’s warnings, he was humiliated to be related to Acheron. The fact that they were identical in looks made it all the worse for him, and he was mocked throughout his life for it. Because of his own isolation brought on by being Acheron’s twin, he hated Acheron with a passion.
Unfortunately, their lives were tied together by Acheron’s mother Apollymi. So when Apollo killed Acheron, Styxx died as well. And when Acheron returned to life, Styxx was also brought back.