Atrus

Atrus is the son of Gehn and husband Catherine, he is also the father of Sirrus and Achenar .  At the beginning of your your journey through Riven, Atrus hands you his personal journal and a prison book, which is used to trap Gehn and free Catherine. 

Gehn

Gehn is the father of Atrus,  and has lost his sanity some time ago.  Gehn wrote the age of Riven and controls its inhabitants.  He  has suffered through much pain, he lost his father in some horrible way, his wife, Keta is also gone.

Catherine

Catherine is the wife of Atrus, mother of Sirrus and Achenar and "leader" of the Moiety people. You are sent to Riven to free her from Prison Island  where she is imprisoned by Gehn. 

Nelah

Maybe one of Catherine's close friends or loyal servants.