The Chosen One

The Chosen One is who was the only one who could assemble the cup. When the cup was broken, Anubis made an agreement with Amneris that once every 25 years, on a special date, at a special time, a member of Amneris' bloodline would be able to reform the Cup of Ankh. This person would be born on the seventh hour of the seventh day of the seventh month. It was originally believed that it was Joybut in the finale it was revealed that Joy was born on the right day, but at the wrong hour. The true chosen one is Nina.

Joy
All the way through the series, The Secret Society believed that Joy was the Chosen One because she was born on July 7th, at 7:00 p.m. Little did they know, that that 7 p.m. is the 19th hour of the chosen day. Not 7th. In the later stages of the series the Sibuna gang believed it too. During the ceremony she couldn't assemble the cup and gave up.

Nina
Nina, the Sibuna leader, was not intended to be the chosen one up until the series finale. She was the chosen one because she was born on the true seventh hour, 7am not 7pm, in the seventh day of the seventh month. She successfully assembled the Cup of Ankh. Fabian had also mentioned once that he thought it was Nina not Joy who was the true chosen one.