|“||To everyone who went against us, we will bare our fangs and retaliate. To everyone who lends their hands to us, we will grant our blessing to them. To our opponents, we will do to them what they have done to us. In the future, I hope that we can be linked in a friendship one day.||”|
–Rimuru, declaring his ideals and goals that would be the foundation of Tempest.
A partner and best friend of the True Dragon Veldora Tempest, Rimuru is the founder and King of the monster country Tempest of the Jura Forest. He is regarded as one of the strongest Demon Lords among the mighty Eight Star Demon Lords and also the only proper Great Demon Lord currently.
Rimuru initially appears as a blue-bodied slime, but upon becoming a Demon Slime, his body turns silver with streaks of gold.
As a slime, since his body is androgynous, anything he mimics takes on an androgynous form of the respective creatures, while still bearing huge resemblances to the individual creatures as well.
His mass also grew over time, allowing him to accommodate larger mimicry forms without using extra magic to compensate for the additional mass. Towards the beginning of the story, he only has enough mass to appear as a human child, but by the time he becomes an Ultimate Slime, he has enough mass to appear as a 16-year-old.
In this form, Rimuru possesses silver (blue in the Light Novel, Manga, and Anime) hair. There are no visible sex characteristics; however, he is often mistaken for a young girl due to the fact that the original form belonged to Shizue Izawa. But when he wears the anti-demon mask he kept as a memento from Shizu, his speech pattern can also make him seem like a prepubescent boy to others.
Unlike Rimuru's other Mimics, his hair changes color to one similar to that of his original slime body. It is unknown if this is just for convenience's sake, but it may also be possible this is because he was once human before and has a special connection to the form.
Since his slime body is equal in mass to his human form, he fills this body with solid shape instead of black fog. Without the black fog, there is no additional magic consumption required to maintain the form. There is no discomfort when he assumes this form. If he uses the black fog, he can also assume the form of an adult.
His fine motor capabilities improve when he is in his human form, and he finds it easier to move in than his slime form. Even though it appears to be human, this form has no need for breathing, excretion, and reproductive organs. However, he can freely eat, taste, and drink. 
In his human form, he wears a black (blue in the Manga and Anime) fur coat and a scarf. Under the coat, he wears the kimono and pants that Shuna had made for him. He also wears the steel thread underwear he makes himself. He often wears Shizue’s old mask to hide his demonic powers.
Since eating the Orc Disaster, his human form grew a bit. He now assumes the form of a pre-teen with feminine features. In this form, he is around 150 cm in height. In addition, he has obtained a devil transformation.
Upon becoming a True Demon Lord and changing his race change to Demon Slime, Rimuru has been depicted wearing a black overcoat. His nails also appear sharper and his overall human form appears slightly older.
Upon evolving further into an Ultimate Slime, Rimuru's human form ages to 16-17 years old. This form is described by Velgurind as appearing gender-less (as she saw it nude upon creation) but with dangerously cute features. By simply willing it, Rimuru can generate clothes from his own mass and magic power.
He has a kind heart and a quirky personality. Even though he is quite laid back and wants to avoid troublesome situations as much as possible, he actually works very diligently when there is a task needed to be taken care of.
With the heart of a hardworking salaryman and a beloved senior co-worker, Rimuru has a hard time rejecting someone if they plead him to help, leading in him often ending up with more work than he can handle, which his boss from his previous life used to reprimand him for.
Being the very Satoru Mikami himself, he also believes himself to be a KY-person (Kuuki Yomenai) who makes jokes at the wrong time.
Rimuru tends to react very severely when there's a danger to his close ones, such as his countrymen, subordinates, (possibly due to the soul-link he shares with them), good friends, and students (he can't stand children suffering). One might say he overreacts to these cases, but it simply shows how deeply he cares for them, as a stark contrast with his usual cheerful self.
Unlike many other shounen protagonists, he has no problems with decimating any forces who dare to harm his subordinates and his country, regardless of their numbers, realizing full-well about the deed he's done.
After a mishap of subsidies, Rimuru tends to quickly revert back to his laid back self and mentally (or even in actuality) scratches his head at the scenery he caused.
He is described as cautious and cunning by other fellow Demon Lords, unaware that it is simply a means of him trying to prevent troublesome situations from going out of hand. As he progresses through the story, he slowly becomes more and more like an actual Demon Lord, and as he adjusts to the rules of the new world and understands the capabilities of his subordinates, he begins to let them handle tough situations on their own instead of voluntarily diving in first to patch stuff up.
He seems to have a lot of knowledge about video games and manga.
- The Goblins: Rigurd, Rigur, Gobta, Gobute, Gobuzo, Gobucchi, Gobutsu, Haruna, etc. Along with the Direwolfs: Ranga.
- The other Goblin tribes: Ririna, Regurd, Rugurd, Rogurd, and several hundred other goblins.
- The Ogres: Benimaru, Shion, Hakurou, Souei, Shuna, Kurobee.
- The Orcs: Geld, Yama(s), Oka(s), Hora(s), Umi(s), Kawa(s), Ko(s), Mori(s), Kusa(s), Suna(s).
- The Lizardmen: Abiru, Gabiru, Souka, Touka, Saika, Nansou, Hokusou, and several dozen lizardmen.
- Zegion & Apito.
- Gozurl, Mezurl.
- Kumara (Coccus, Raiko, Getto, Yoda, Mink, Byakuen, Enchou, Igami), Adalman, Albert.
- Black Corps Demons: Testarossa, Ultima, Carrera, Moss, Veyron, Venom, Esprit, Cien, Zonda, Agera.
- The Dragon Guardians: Zephyrus, Boreas, Euros, Notos, Wenti.
- Rimuru's original coloring in the original Web Novel is white instead of the color blue like in the Light Novels, Manga, and Anime.
- He, along with Shuna and Shion, are voted as the three major idols of Tempest.
- Rimuru likes to fantasize about female Elves.