Zain has been best friends since he first met River thousands of years ago. He has been with her side by side, almost literally. To save him River allowed him to summon his soul into her body so that he may live on with her and they could always be together. And that would be their downfall. He was always there helping her and River got sick of him telling her what to do. And when Zain hit River's last button she forced him out. Out of hurt, he went to Astrid for help, to get revenge on River. That would be a mistake since River took her anger out on him, killing him.  So he got to watch his world and family fall apart and he couldn't do anything, he was just a  ghost. But he could do something. He came in contact with Ivory when she died and together they both convinced Greyson to take his mother on and correct what should've happened thousands of years ago.