Otherworlder ｢異世界人, isekaijin, lit. "Other World People"｣ is a term that refers to humans that come to the Central World from other worlds.
Summoned[]
Summoned ｢召喚者, shōkansha｣ are the most common type of otherworlder, brought over through summoning rituals. In the process of crossing worlds, the summoned is exposed to a large concentration of magicules dense enough to kill any normal human. Only those that bear a strong soul and mind can endure the strain of the journey.
From the massive energy taken in by the summoned, Unique skills and other special abilities are born as a response to their desires. With these mouth-watering powers, and a strong body that no human can achieve without undergoing grueling training, otherworlders became highly sought after by human nations to serve as a weapon against monsters, and are also employed as bodyguards for nobles and royalty. For this reason, a locking curse is placed on the soul of the summoned to prevent them from fleeing.
There are very few nations capable of summoning otherworlders due to the sheer costs and level of preparation involved. Even meeting the ideal conditions for a proper summoning, the chances of successfully calling an otherworlder is less than 1%. There's also an inconvenient cooldown in place that prevented the ritual from being performed continuously in quick succession. Due to knowledge circulated by Leon Cromwell, however, a less strict and simplified variant of the summoning ritual was eventually born. While the success rate remained low, it could be used continuously with a reduced cooldown period, but the consequences were that it often resulted in the summoning of children. These children usually had no skills and were left with only their vast magical energy, which, having failed to be converted to skills, would eventually destroy their bodies within five years. Upon Rimuru Tempest becoming a Demon Lord, the ban on the practice of summoning otherworlders was re-enforced, bringing the already illegal practice to a halt even behind closed doors.
Stray[]
Strays ｢来訪者, raihōsha, lit. "Visitor"｣ are otherworlders that arrive by sheer accident. There is no clear cause or pattern known for why this happens, but in general, it's considered a one-way street and the phenomenon is not known to happen in reverse.
Strays are drawn to the central world through dimensional cracks that spontaneously open in other worlds, drawing in any unfortunate passerbys. They don't get called to fixed locations like summoned and can appear virtually anywhere, including dangerous environments depending on their luck. If they manage to survive whatever dangerous predicament they may face, they can live however they please without worrying about being enslaved.
Since there is no strict standard for who can cross over, strays tend to be on the weaker side compared to other types of otherworlders. Kaoru Yoshida, who is a stray, is virtually no different from the average human. On the other hand there is Hinata Sakaguchi, who obtained two unique skills upon her arrival. As shown with these examples, the strength of strays is very random and can be all over the place, with most not guaranteed to ever obtain unique skills.
Reincarnated[]
Reincarnator ｢転生者, tenseisha｣ or Transmigrant, refers to individuals that arrive with no body and just their mind, being reborn as an inhabitant of the central world. Reincarnation tends to happen randomly. Although there are those among the reincarnated who still remember their past life, to have reincarnated from a different world is exceedingly rare. Moreover, one would normally be reincarnated as a human. For someone to be reincarnated as a monster, and from magical energy itself, was something that remained unheard of. The only known example of this is Rimuru Tempest.
