"The Secret of The Systems"
[Y-you can sign an agreement before your part-time contract ends or if your performance is good, w-we will always summon you after your life ends.]
Some souls became hosts, while those who weren't good at dealing with a new environment every so often could be systems instead.
It's just that those who became systems usually came from novel worlds that turned into real worlds.
Yes. Monica was once a novel character like Isabella, but after she let go of her previous life, she started a new life as a newbie system.
Monica knew that she was once a person living in a newly born novel world, just like Isabella.
But as for what kind of life and grievance she faced back then, she didn't remember anything.
Obviously, whoever turned Monica's soul into a system didn't want Monica to feel sad or bad because of her past life.