My crackpot theory is that Melina is a creation of Marika, (datamined/internal name is "MaricaofDaughter") with the sole purpose of burning down the Erdtree as seen in game. It appears that Marika can perhaps fuse or absorb people into her, "O Radagon, leal hound of the Golden Order. Thou'rt yet to become me, thou'rt yet to become a god. Let us both be shattered, mine other half". Radagon and Marika seem to be two individuals who became one.
Maybe Marika absorbed the Gloam-Eye Queen after her defeat by Malekeith, and later used it along with a part of Ranni's lost soul (lost in death) to create Melina around the time of shattering the Elden Ring.
Hence her gloam eye as seen in the Frenzied Flame Ending, along with Ranni's/Rennala's blue eye, and Radagon's red hair.