Otherworld Summoning

Otherworld summoning is for calling heroes to defeat Demon kings. This can happen one of two ways: transportation or reincarnation.

Transportation
Some or more poor schmucks are pulled from Earth and get skills; completely without their consent. It's currently unknown if they CAN be sent back home, or even to the moment they left; considering the longer they're gone, the more likely they'll be declared dead or even locked in a lunatic Asylum when they explain being in another world. This happened with the four heroes of Deramis: Touya Kanzaki, Miyabi Kuromiya, Nana Mizuoka and Setsuna Shiga.

Reincarnated
With the former a deceased soul is offered a chance to be reborn with skills chosen abilities; they can even alter what they will look like and even age. They can even sacrifice factors of themselves to boost the skills they can use. Kelvin notably erased his memory of Earth in exchange for his great powers. (If one could sacrifice years they've lived AKA age, this would be a major bonus if an elderly soul was selected) Plus the more MP offered by a human for this summoning, the more powerful the reborn person will be; Kelvin gave up all but 1 MP and ended up giving Rion great abilities.