Otherworld Summoning

Otherworld summoning is for calling heroes to defeat Demon kings. Since there are limits on the people born in this world that Earthlings don't.

Transportation
This takes a crap ton on magic points, but is the only method to summon more than one; if you want a party of Heroes to the dirty work.

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; though this mostly related to his identity, akin to amnesia. 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.