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 his human form, Rimuru possesses long light blue hair and golden eyes. 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. He wears a blue 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. 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.
Rimuru tends to react very severely when there's a danger to his close ones, such as his countrymen, subordinates, 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.
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.
Using Great Sage, he can quickly read a book by copying it completely. Simultaneous utilization of Great Sage and Gluttony allow the high-speed copy and review of the magic books that he holds in his hands. With that, he can postpone checking the content of the book for a later time. Unfortunately, it is not possible to be able to use magic just by copying books. But, taking and copying it, he's able to make the index of the magic books that he wants. After this, it's necessary for him to study them one by one. Taking that into consideration, it can be said that Rimuru holds a great amount of knowledge regarding magic as well as being speculated to be able to use most of the magic in those books, if not all of them.After evolving into a Demon Slime, the ability to freely change between material and spiritual bodies was added. As a result of the evolution, his total magical energy greatly increased. Compared to the original amount, it was ten times as large.
After fighting the Guardian of the Labyrinth, Rimuru has the ability to create golems. The abilities of the Golem can be added through the possession of a spirit or a demon as Rimuru made them inhabit the Golem. The outward appearance of the golem is created according to his imagination.
Using Manas: Ciel, Rimuru can copy space manipulation techniques simply by seeing it, such as when he copied the Unique Skill Restrainer which belonged to Gozurl. Like this, Rimuru can also copy practically any other Skill or Magic as long as he has seen it.
After the fight with Yuuki, Rimuru gained the ability to use the Instantaneous Movement which lets him travel across Space-Time at will. Using this ability, Rimuru was able to travel to his old world where he saved his past self Mikami Satoru from death. He was also able to return from the place beyond time where he was exiled to by Yuuki. The ability as described is the ability to reach the desired point instantly across space and time