When Thirteen was born, he didn't become the System of Cannon Fodders right away.
All Systems were allowed to choose their "Chosen Ones," and he chose a unique individual as his First Host.
Thirteen's first partner was a very eccentric creature who liked mischief.
That creature also liked to create chaos and valued freedom a lot.
Thirteen had spent hundreds of years with him, and through their journeys, the System, who was only starting to know more about the world, gradually noticed a few things.
Since his First Host was neither good nor evil, Thirteen was able to see the many faces of the world through his eyes.
At first, he was indifferent to the things that he saw. But after hundreds of years, he was starting to question many things.
Of course, his host was more than happy to answer his questions. The human hadn't thought of Thirteen as a tool but as a friend who suddenly appeared in his life when he was all alone in the world.