She is the spirit of the Lethe River which flows drowsily through the Underworld via the slumberous chambers of HYPNOS.
Souls due for reincarnation must drink from the sluggish waters — this will erase their memories of all previous lives, presumably to avoid cluttering their minds with past glories and old sorrows. This, of course, is why no-one remembers having lived before. Shame they couldn’t let us remember the good bits though.
LETHE is the daughter of chaos agent ERIS, and we all know how much consternation forgetfulness can cause.
Okay, that’s all we can remember... sorry... zzz...