There's a lot of worlds, a whole lot of fucking worlds with each one having a unique system and characteristics like having magic, martial arts, leveling systems, heck you can even find a NTR themed world.
However, despite the variety of all of these worlds, every single one of them has a cycle in which every living being in a world undergoes the "cycle of reincarnation" in which if that living being dies, they would be given the chance to reincarnate into another world.
There are some special cases in this cycle and they are called a "Figure" in which they are privileged beings that are given the chance to reincarnate with their past memories and abilities intact. Figures are beings that left a great impact in their world whether if it's evil or not before dying.
And governing the "cycle" is a being so divine that even the word "God" is an understatement for its majestic presence and for some unknown reasons, this same being suddenly thought of an outrageous idea which is...
"What if I tossed 2 figures in one body?... Wait- scratch that. Let's make it 8!"
Thus, a fated human is born with 8 different past lives and memories. Pretty insane right?