Ryssa was the older sister of Acheron and Styxx. She had a son Apollodorus with Apollo, and was killed when she was twenty-seven. Despite often causing Acheron pain, her motives were always that of kindness. Ryssa was seven years older and seemed to be the only one who played with him before he was taken to Atlantis.
When Acheron was taken to Atlantis to be a sex slave, Ryssa wrote to him and thought that her brother was writing back when in fact he was not. She was appalled when she arrived, having not realized what was happening to him. She thought that he had escaped from the beatings their father gave Acheron and was happy and healthy.
With the help of her servant, Ryssa rescued Acheron and took him to the summer palace and slowly he started to trust her. However, their father showed up and dragged him back to Atlantis just three days before he was supposed to be taken to a refuge Ryssa had found for him.
Ryssa next saw her brother when her uncle died and they went to collect him. Acheron talked back to their father and was thrown out into the streets with Ryssa's cloak as his only belonging.
She found Acheron again some years later in a brothel but he was eventually arrested without Ryssa's knowledge because people had been making fun of the heir to the throne Styxx, twin brother to Acheron. However, Styxx had blue eyes, as opposed to the grey ones that Acheron had. Ryssa found out where her brother was being held, in the lowest dungeons of the palace. His cell was barely big enough and he was forced to bend almost double. He was eventually freed, but kept tied to his bed when Styxx fell ill because Acheron was starving himself.
Ryssa procured him slightly more freedom after she was chosen to be given to Apollo. She said that she would scar her face so that he would not want her if Acheron was not treated better.
Ryssa initially learned of the relationship between Artemis and her brother Acheron, but kept quiet about it. She tried to tell her father of it in order to prevent him from castrating Acheron, but no one believed her, and Artemis thought that it had been Acheron who told, and beat him for it. Artemis eventually found out it was Ryssa and said it was only because Ryssa was pregnant with Apollo's child that she was still alive. Artemis later erased Ryssa's memory of the relationship.
After Acheron was given his god-powers back, Ryssa showed up with a present for him. It was a bracelet with a charm on it. The charm was the symbol of Acheron's mother Apollymi. Ryssa was upset when Acheron told her the symbol was Atlantean because she thought that she had caused him pain. Reading her thoughts, Acheron understood for the first time how much she cared for him. Earlier, it was only for Ryssa that Acheron promised to wean himself off drugs.
Acheron was also good at calming Ryssa's son.
Ryssa died along with her son on her twenty-seventh birthday. It was because of Ryssa's death that Apollo placed the curse on the Apollites. Acheron had been drugged by Artemis at the time and was unable to save his sister and nephew.