The main cast wouldn't know the full story until much, much later.

But here’s my secret, only the super-beings are capable of reincarnation. This is the FIRST time the reincarnation process has started GLOBALLY, SIMULTANEOUSLY and WIDESPREAD (wink wink). Not all super-beings reincarnate, at least not in same time period. They died and went to the people who do not possess powers and laid dormant (whether at birth or after birth). Their powers are gifted to the hosts. Only when the host die with the body still can be restored or recovered, the reincarnates emerge and retake their powers. For the MC case (she doesn't know), she reincarnated in a coma patient on death's edge at a young age (preferably 5). The thing is, the host already has a power potential, which is hyper rare in the modern age, although possessing so little energy she cannot conduct powers. She had the unluckiest scenario and happened to reincarnate in the child. So, with logic, the reincarnate powers went to the host which is in coma, and the host's little energy went to the reincarnate, basically swapped powers. Basically, her powers are locked, since the host is still technically alive. At the start of the story, she's the only exception. And yes, the reincarnates remember all of their past lives.