Brief explanation Red: You were a regular person in our world, however one day you began to see memories of a past life. While still sorting things out, powers started to manifest in you, powers and items that "you" used in the memories. Soon, after you started adjusting to your new life, you were contacted by a butler named Kuro or a maid named Kura. They told you that all of reality was at risk, and that your powers had been anifested was to save it.
After giving you some time, you were brought to a room filled with others whose memories have also returned. In this room, they explained that a being known as the Supreme Darkness was consuming the worlds that exist. It was originally being kept in check by Kuro and Kura's master, the Emissary, but he recently vanished. Now, they need you to help save him by reclaiming your past lives to grow stronger, fight pieces of the darkness throughout the worlds, and help find their master.
However, you decided you didn't want to come along. Maybe you were scared, maybe you wanted to test your powers first, maybe you didn't want to be stuck in this group, probably you didn't want to be stuck in this group. And that was fine. They gave you a nice room to stay in that would change itself to suit your tastes, it could provide you anything or anyone. And you spent your time there for the first two missions. But the second mission is about to end, and for one reason or another you've decided it's time to stand up and fight.
When the current battle in the other thread ends, and the others return to the base, you will join the others and go with them on their next mission.
But you're gonna have to wait a bit, otherwise it will end up feeling too unnatural.