- Web Novel
Geld, your Ravenous is a powerful unique skill, I’ll grant you that. But you’re not a predator. You’re a scavenger. You can eat anything, which is incredible, but I’m adapted for catching and consuming. And that gives me the advantage. If we’re gonna keep eating at each other, I’ll be the one to gain new skills from it first. My Predaceous instincts will it to be done! I can analyze and obtain skills even from living creatures, but you, Geld—you can only steal them from their corpses. And that’s what will decide this.Rimuru, to Orc Disaster Geld
Partner and best friend of the True Dragon Veldora Tempest, Rimuru is the founder and King of the Jura Tempest Federation in Jura Forest. He is one of the Demon Lords of the Octagram. He lives in Rimuru City, the Capital city of the Jura Tempest Federation
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 each time he consumes a foe powerful in relation to his current strength, his size in human form shows a notable increase.
In this form, Rimuru possesses blue-silver 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-magic 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. This might be related to Shizu "optimizing" her body for spirit possession by Ifrit for several decades.
Since his slime body is equal in mass to his human form, Rimuru 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 monster aura.
Upon becoming a True Demon Lord and evolving his race from Slime to Demon Slime, Rimuru has been depicted wearing a black overcoat. His nails also appear sharper and his overall human form appears slightly older.
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. Even Hakuro noted during The Rise of the Demon Lord Arc that Farmas had "angered the one creature in the world that should not be riled." During this time Rimuru showed a great rage boiling within him for the murder of his countrymen; though he could suppress any outward expressions of it, he needed a copy of Shizu's mask to suppress his magicules due to his rage unintentionally releasing his power.
He also has a rather cold side to him which is not shown in the manga and anime, as for example in the version of events where the Goblin Riders instead of Rimuru save Shizu and the adventurer trio from the Giant Ants, Rimuru is much more distrustful towards his guests and when Shizu goes on a rampage, he even suspects that it's all a trap to let his guard down so that the adventurer trio can backstab him. Rimuru also casually mentions that he'd be able to just kill off his followers if they decide to rebel against him.
The greatest flaw of Rimuru's personality are his anxiousness issues that are present in all three media. After the Orc Disaster was defeated, Rimuru seemed sure that the Kijin wouldn't follow him afterwards and he was immensely surprised when they did. When Kenya Misaki became a Hero candidate, Rimuru worried if he'd hunt Rimuru down if he found out that he is a Monster. Finally, Rimuru was fully expecting to be considered a traitor after revealing his identity as an Otherworld Reincarnator, but contrary to his expectations his followers remained steadfastly loyal to him.
Another personality flaw in Rimuru is his naivete as a world leader. He was reportedly easy to persuade during diplomatic negotiations, which some rulers tried to take advantage of. King Gazel also recognizes this flaw and informs Rimuru he is too humble and softspoken as a leader. Rimuru's desire to be pacifist towards humans eventually has dire consequences for his people, which he personally shoulders the blame for. This stops being a problem after Rimuru becomes a Demon Lord, giving him a foreboding title to make would-be troublemakers fear him; even after they get to know him, his status as Demon Lord still keeps them wary.
During the battle, Rimuru often tends to put himself down while expecting his enemies to be stronger than they actually are. While it causes himself to sometimes be too passive and too fixated on hiding his powers, he also is very unlikely to become cocky during a battle as a result, so it's hard to say if this trait is a net positive or net negative.
Despite his power and abilities, Rimuru's greatest asset is his cunning and shrewd decision-making. Rimuru has survived against dangerous opponents such as Milim Nava and Hinata Sakaguchi, who could have easily overpowered him prior to his awakening as a True Demon Lord, simply by outsmarting them. Rimuru prefers diplomatic solutions and will avoid conflict whenever possible, but isn't afraid to wage war when necessary, as shown when the Kingdom of Falmuth invaded.
- The Goblins: Rigurd, Rigur, Gobta, Gobute, Gobuzo, Gobucchi, Gobutsu, Haruna, etc. Along with the Direwolfs: led by 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 and Apito.
- Rimuru seems to have a lot of knowledge about video games and manga.
- A running gag is that everyone who meets Rimuru is surprised that a slime is in charge. Later this changes to everyone being surprised how kind and humble he is for a Demon Lord.
- He seems to have an elf fetish.
- Rimuru is shown to have a perverted side, which is balanced out by his own self-conscious reprimands and gentlemanly conduct to women.
- Rimuru often bathes in slime form with Shuna and Shion at the hot spring.
- In one OAD, he designs the swimsuits for Haruna, Shion, Shuna, and Souka.