Most isekai anime follow a similar formula that focuses on the main character reincarnating in a new world and slowly leveling up and growing in power. While some anime play around with this basic premise, several trends can be commonly found in isekai anime.
Two of the most popular isekai anime right now are That Time I Got Reincarnated As A Slime and So I'm A Spider, So What? Although each anime has carved out its own group of dedicated fans, the two series share more elements in common than differences that set them apart.
The Main Protagonists Are Reincarnated After Dying On Earth
The main feature that all isekai anime have in common is that the main characters are suddenly transported to a new world. The most common causes for reincarnation involve the characters either being transported into a video game or dying and reincarnation into a new world.
In That Time I Got Reincarnated As A Slime and So I'm A Spider, So What?, the main characters are reincarnated after dying violently on Earth. Satoru Mikami reincarnates after he's stabbed by an unknown assailant in Tokyo, while the unnamed spider dubbed "Kumoko" by her fans reincarnates after an explosion at school kills her and her classmates.
The Protagonists Are Monsters
One aspect that sets both anime apart from most other isekai titles is that their main protagonists are monsters. Typically, the main protagonists in isekai anime are humans or at least humanoid, but these anime take a different route by casting a slime monster and spider monster as the heroes of their respective stories.
After Satoru Mikami is killed, he is reborn as a slime monster in a monster-filled dungeon. When he encounters the Storm Dragon Veldora, Satoru takes the name Rimuru Tempest and sets off to explore his new world. Similarly, unlike the other students in her class, Kumoko reincarnates as a spider monster in an even more dangerous dungeon, The Great Elroe Labyrinth.
They Use A Cute Art Style
Despite the monstrous forms of the main characters, both anime use a cute art style to portray Rimuru and Kumoko. This decision has the effect of making Rimuru and Kumoko less threatening and therefore more likable to a younger audience.
In his slime form, Rimuru is a cute blue blob. When he blushes his cheeks turn red, and due to his body composition he can transform his body into strange shapes such as a question mark or exclamation point. Meanwhile, although Kumoko appears as a terrifying monster to humans, in her own mind, and to the audience, she appears like an adorable white spider.
They Both Feature An Administrative System
When Rimuru and Kumoko reincarnate into their new forms, they are initially extremely confused, especially Kumoko. Thankfully, their new homes both feature an administrative system that helps them to understand their surroundings and the abilities that they acquire upon reincarnating.
Rimuru reincarnates with the skill Great Sage, an internal computer that explains the world around him and easily becomes his most useful non-combat ability. While Kumoko's administrative system does not start out as useful as Rimuru's, over time it becomes more detailed as she levels up. Additionally, Kumoko eventually gains contact with the all-powerful Administrator D, who gives Kumoko advice and insight over the course of the series.
The Main Characters Are Overpowered
It's a well-known fact that the main protagonists in most isekai anime are insanely overpowered. Even compared to the heroes in most shonen anime, isekai protagonists start out with abilities that quickly allow them to outpace their enemies and become the most powerful beings in their new worlds.
At first, Rimuru has few abilities outside of Great Sage and Predation, but these two skills are all he needs to quickly level up. By the end of the first part of season 2, he attains the status of a Demon Lord, making him one of the strongest beings in the world. While Kumoko's path to power takes longer than Rimuru's, she eventually rises to be the Nightmare of the Labyrinth, and her power is matched only by the Demon Lord Ariel and the strongest of dragons.
Both Involve Political Intrigue
Much like the worlds they left behind, the worlds that Rimuru and Kumoko now call home are full of political intrigue. Both protagonists are forced to navigate a complicated series of diplomatic negotiations, border wars, and power struggles all while trying to justify their existence as monsters.
For Rimuru, his biggest obstacles concern securing legitimacy for his nation of monsters, the Jura Tempest Federation. A large portion of the series is dedicated to his efforts at establishing relationships with nations like the Armed Nation of Dwargon and his conflict with the Kingdom of Falmuth. To her credit, Kumoko tries to avoid embroiling herself directly in political intrigue, but even she cannot avoid it completely, such as when she gets enmeshed in the conflict between Seriella and Ohts and in the machinations of the Demon Lord Ariel.
They Both Use Narration To Further The Story
Narration is an important storytelling technique that is used in multiple forms of media. It's common for anime characters to use narration to cover events through flashbacks, or as filler to explain events during time skips.
That Time I Got Reincarnated As A Slime frequently uses narration to discuss Rimuru's activities and accomplishments during time skips. This narration explains how his diplomatic relations are advancing with the Armed Nation of Dwargon, along with the progress of his different trade negotiations. Meanwhile, in So I'm A Spider, So What?, Kumoko narrates to cover the time skips between her jumps in power levels, as this helps to explain how she increases in power so much between episodes.
They Depict Humans In A Negative Light
Both That Time I Got Reincarnated As A Slime and So I'm A Spider, So What? tend to show humans in a negative light. While this is not unique, their decision to instead portray monsters as either good or neutral characters is novel and sets both anime apart from other series in the isekai genre.
Aside from Shizue and the children Otherworlders, the humans in Rimuru's new world are not particularly friendly. The humans of the Kingdom of Falmuth are depicted as manipulative and cruel, while even the people of the Armed Nation of Dwargon are portrayed as cunning and cold. Similarly, while Kumoko's classmate Shun is depicted as a noble Hero, many of the other humans in Kumoko's new world are brutal and deceptive, especially her former classmate, Hugo.
Both Include Intense, Abilities-Based Battles
There is no shortage of action in That Time I Got Reincarnated As A Slime and So I'm A Spider, So What? Both anime feature fast-paced, technical battles involving a rotating list of complex skills and abilities that make for highly entertaining and intricate fights.
Rimuru fights a number of tense battles over the course of the series, the best of which have to be his fight with the Orc Disaster Geld and his duel with Hinata Sakaguchi. Although it hardly seems possible, So I'm A Spider, So What? manages to cram in even more battles than That Time I Got Reincarnated As A Slime, as hardly an episode goes by without Kumoko being forced into a life-or-death battle against a terrifying monster.
The Protagonists Use Humor To Help Them Cope With Difficult Situations
Rimuru and Kumoko live in worlds that are dangerous and scary. They are full of terrifying monsters and Demon Lords and ruled by powerful kingdoms, yet somehow both Rimuru and Kumoko are able to keep a positive attitude throughout their dangerous ordeals.
Rimuru is able to keep up his spirits by joking around with his friends. The Kijin, Hobgoblins, Lizardmen, and Dwarves that live with him in Tempest become Rimuru's surrogate family and he feels comfortable being himself around them. Although Kumoko does not have friends she can laugh with, she still manages to amuse herself, especially once she unlocks the ability to split up her conscience into separate bodies.