The rise of isekai themes in anime can be reliably tied to the success of 2018's breakout series That Time I Got Reincarnated As A Slime. Isekai anime involve stories where a protagonist is reincarnated into a new and strange world, and that is exactly what happens to Satoru Mikami when he is killed in Tokyo and reincarnated as a weakly slime monster in a bizarre fantasy world.
That Time I Got Reincarnated As A Slime follows Satoru as he slowly levels up and unlocks new abilities and the secrets of the strange world he now calls home. Over time, he takes a new name, Rimuru Tempest, and makes many friends that help him on his journey to establishing a home for monsters like himself. Each character brings something different to the table, with varying degrees of likability.
Hakurou
Hakurou is one of the surviving members of the Ogre village that was destroyed during the Orc invasion of Jura Forest. Before being named by Rimuru, Hakurou was depicted as an old man, and after being named and evolving into a Kijin he appeared younger and more agile.
Hakurou was the greatest swordsman in the Ogre village and was the one who originally trained the King of the Armed Nation of Dwargon, Gazel Dwargo, in the way of the sword. He trains the other warriors of the Jura Tempest Federation in swordsmanship and takes his role as a trainer very seriously. He is quiet, thoughtful, and calm, but when his students or honor as a swordsman is threatened he can become menacing and full of subtle rage.
Shuna
Shuna is the former princess of the Ogre village and Benimaru's younger sister. Shuna is a gifted magic user but is rather shy and humble when it comes to using her powers. Rather than serving as a fighter in Rimuru's army, she prefers to work as his advisor and personal assistant.
She is an amazing cook and seamstress and oversees many of the operations of the Jura Tempest Federation, specifically those having to do with cultural affairs and diplomatic relations. She is quiet, thoughtful, and cheery, but also very competitive, and fiercely protective of Rimuru. Shuna is the most agreeable and pleasant of her Ogre kin, and her youthful and bubbly personality makes her stand out from many other characters in the series.
Shizue Izawa
Although she only received a limited amount of screen time in the anime, Shizue Izawa played an important role in the storyline of That Time I Got Reincarnated As A Slime. Like Rimuru, Shizue was an Otherworlder from Japan, and the two bonded over their shared lineage. Unfortunately, Shizue was badly weakened as a result of the spell cast by Leon Cromwell that summoned her from Japan, and as a result, she was possessed by the Flame Giant Ifrit to sustain her life force, slowly killing her in the process.
Shizue was a skilled warrior and a dedicated teacher to the young Otherworld students at Freedom Academy. She was a kind, thoughtful, and brave character who left an indelible impression on Rimuru, inspiring him to find and help Otherworlders and Monsters alike in this new world.
Gobta
When Rimuru saved the Goblins of Jura Forest from the Direwolves, he named them and spurred their evolution into Hobgoblins. One of those Hobgoblins, Gobta, retained his Goblin appearance, and soon cemented himself as a fan-favorite character. Gobta may seem like a fool, but he is actually very competent.
He quickly rose through the ranks to become the captain of the Goblin Riders and distinguished himself as a martial artist under Hakurou's tutelage. Like Rimuru, he is very trusting and loyal to his friends, and more perceptive and intelligent than he seems. He stood by Gobzo when he was false accused by Kirara Mizutani because he was able to see through her lies, and also because he had the utmost faith in his friend's character
Shion
Rimuru's secretary and primary bodyguard, Shion, is another Kijin who pledged her loyalty to Rimuru after fleeing from the Ogre village. Shion has made it her personal responsibility to ferry Rimuru around and protect him from any potential enemies, a role she is more than capable of fulfilling as one of the strongest fighters in the Jura Tempest Federation.
While she is rather clumsy and a little dense, she is also one of Rimuru's most passionate companions and does not condone any insults or threats issued toward Rimuru or her friends. She always gives maximum effort no matter how small the task, and prefers to tackle problems head one, especially if she is allowed to use her fists or giant sword in the process.
Souei
The leader of the Shadow Squad of the Jura Tempest Federation, Souei is a Kijin from the Ogre villain who has since become Rimuru's faithful shadow. He serves as Rimuru's chief espionage agent and takes his position very seriously, often finishing Rimuru's sentences and predicting his orders before they are fully delivered.
Souei is serious and competent and demands excellence from all of his subordinates. He is a ruthless operator and does not hesitate to cut down enemies of the Jura Tempest Federation or any who threaten Rimuru or his friends. While he is very focused when on a mission, he also enjoys relaxing in his downtime, especially with his best friend, Benimaru.
Ranga
Rimuru's faithful subordinate, Ranga, is the leader of the Tempest Wolf Clan and the former son of the leader of the Direwolves. Ranga is one of Rimuru's most dedicated supporters, and never questions or doubts Rimuru's orders or intentions. He normally resides in Rimuru's shadow, so that he can always be close by to carry out Rimuru's orders.
When he is not in Rimuru's shadow he is usually acting as Rimuru's steed, ferrying Rimuru about at breakneck speeds. While he may look like a massive Direwolf, deep down he has the heart of a loyal dog, as seen through his manner of licking Rimuru, wagging his tail when he is happy, and living to serve and protect his master.
Milim Nava
Also known as the Destroyer, Milim Nava is one of the strongest and oldest Demon Lords currently in existence. Milim may act like a carefree and immature young girl, but in actuality, she is a cunning and dangerous fighter. More than anything, Milim is searching for activities and people to help stave off her growing sense of boredom.
Prior to meeting Rimuru, Milim was considered to be a walking catastrophe who had no allegiance to anything or anyone. Rimuru becomes Milim's first true friend and his acceptance of her despite her faults leads her to befriend Rimuru and his followers in the Jura Tempest Federations. While she is a dangerous companion, she has calmed down considerably thanks to Rimuru's and his followers' care and friendship.
Benimaru
The older brother of Shuna and strongest of the Kijin that escaped the Ogre village, Benimaru is a loyal and competent companion and Commander-in-Chief of Tempest's military forces. While he is not as skilled in the sword as Hakurou, as agile as Souei, or as physically brutal as Shion, Benimaru combines the best of all their abilities, making him a truly dangerous combatant on the battlefield.
Benimaru is very confident in his abilities, and only follows those he deems worthy of his services. He is originally hesitant to assume the role of leader of the survivors from the Ogre village, but he grows into his role and becomes a reliable leader. While he acts hot-headed at times, Benimura is solidly positioned as Rimuru's right-hand monster and respected for his intelligence, strength, and honesty.
Rimuru Tempest
From the beginning of the show, fans are able to see Rimuru's selfless personality, as he sacrifices himself to save his friends from a mugger. When he is reborn as a slime, he effortlessly makes friends with other monsters in the new world he now calls home. Rimuru is honest and loyal and genuinely wishes the best for monsters and humans alike.
He is very humble despite his god-like powers and has a carefree and positive attitude. While he grows more cautious and reliable as the series progresses, he maintains a down-to-earth demeanor and is always willing to make time for even his lowliest follower.