The prequel
In the beginning aqua was assigned to guide humans who died you to their next location, but one day when she got mad at this one person for their behavior, she decided to send that person to another world to get her revenge, that person who is now mad at aqua takes advantage of their situation, learns many different skills and relies on their intelligence to become the most powerful adventurer then starts capturing all the top adventures and turns them into monsters, and earns the title of the devil king, and those captured adventures are now his generals in his army. Aqua got into a lot of trouble for this because now almost nobody is choosing reincarnation and the population was decreasing, out of desperation they had no choice but to send more people to the other world to balance it out and if their lucky one of them could stop him by fighting fire with fire, because this was Aqua's fault, she was now tasked with sending the young souls who were willing to go to the other world and provide them with a skill or gadget to assist them for there bravery, but when she offered the choice like a con-artist and was practically tricking them into going there, once again she was upsetting the higher up's, then the day came when she meet kasuma, and then he said the words that surprised everyone,
Kazuma: pick one thing to bring to parallel world, right?
Aqua: that's right
Kazuam: okay, I pick you
Aqua: all right, stand right there and don't leave the magic circle-
Aqua: what did you just say?
They immediately jumped at the opportunity that kazuma gave them and decide to approve his bizzare request, they thought that not only will this young man have the best chance of defeating the devil king, but if nothing else this could serve as Aqua's punishment for causing these problem, and she can only return when she cleans up her mess.