Merrow is Serafina's ancestor. She's one of The Six Who Ruled. Her talisman is Neria's Stone, a flawless, teardrop-shaped diamond.
She is the reason why there are mer, because she asked Neria to allow the people of Atlantis to be able to live underwater when Atlantis was under siege from Abbadon. She hid the truth about Abbadon and Orfeo for a long, long time.