What Agetha needed to do to get her wish fulfilled was actually very simple. She just needed to use her underworld network to track down an item called "Ruby of Ghehena" and that would be it. Ruby of Ghehena was actually a hidden material that could help her to improve her Armour that she got because of her inherent special ability.
Actually even Rein had no idea that this was the case. It was truly a hidden truth as even with his Eye of Truth, Rein was unable to find out about this detail. Actually as Rein was very confident in his Eye of Truth, he actually had a hard time believing that his beloved kill was unable to analyse something like this.
The Ruby of Ghehena was definitely a precious and rare item but by no means it was impossible to find. These gems could be found in mines located in areas with high magical concentration and there were countless such mines present in this world.