Slime
In their normal form — referred to as slime form ｢スライム形態フォーム, suraimu fōmu｣ — Rimuru takes the appearance of a spherical — sometimes amorphous — mass of slightly blue and translucent ooze. Rimuru's initial slime form has a diameter of around 40 cm and it increases as they get stronger across the series, becoming around 80 cm after reincarnating into an ultimate slime.
When using the Body Armor skill, Rimuru's slime form takes on a metal slime ｢メタルスライム, metaru suraimu｣[1] appearance; while their demon slime form sometimes takes the appearance of a golden slime ｢ゴールドスライム, gōrudo suraimu｣.[2]
Human
Rimuru's facial features in human form are nearly identical to those of Shizu, but the long silver-blue hair and deep golden eyes give off an otherworldly divine beauty that sets the two apart. Rimuru can also occasionally be seen wearing Shizu's mask or donning draconic wings.
Initial
Rimuru's initial human form before consuming the Orc Disaster was that of a child that looked no older than six or seven. It had short hair and was typically dressed in a simple outfit made of pelts before switching to the more iconic shirt, pants, and leather jacket combination.
Demon lord seed
After consuming the Orc Disaster Geld, Rimuru gains another 10 cm in height and significantly longer hair, giving the appearance of a child around the age of nine. The primary outfit in this time period consisted of loose black pants and a white and gold shirt both woven from silk mixed with Steel Thread. Over this, they wore a thick black (blue in the anime and manga) jacket and a fur scarf both made from the pelt of the Direwolf Boss.
True demon lord
After the Harvest Festival and evolution into a demon slime, Rimuru gains another 10 cm in height, giving the appearance of a child around the age of twelve or thirteen. During this period the primary outfit consisted of black pants, a black and gold shoulderless shirt, a fur scarf, and a black haori with white accents down the sleeves.
Ultimate slime
After evolving into a True Dragon and subsequently reincarnating into an ultimate slime, Rimuru's human form is that of an androgynous albeit girly 15 to 16 year old adolescent. The main outfit from this point consists of black pants and a black sleeveless long shirt accented with white boots and white stripes respectively. The arms have black detached long sleeves with golden ornaments at the top as well as a golden collar piece around the neck. The white rope-like belt has a metal loop at the back for the purpose of tying the scabbard of a sword. They can also occasionally be seen wearing a cloak over top of this outfit.
Additionally, Rimuru has been seen wearing a more casual version of this outfit with a more simple pair of overly baggy black pants and a white shirt.
Personality
Rimuru is an eccentric and childish person by nature but this shouldn't be mistaken for immaturity and naivety. Underneath the surface, Rimuru is constantly analyzing, planning, and weighing the pros and cons of their actions, taking the smart move in the long term over more rash and emotional decisions. In betrayal of this composed and calculative demeanor, Rimuru is ruthless when it comes to handling people who have aroused genuine fury inside of them. They'll go as far as acts of brutal torture and murder without so much as batting an eye if it can be gotten away with, and they use more subtle methods like intimidation and blackmail when those cards are off the table.
Rimuru is also highly intelligent and takes a great deal of care with their personal image by playing manipulative games, such as lying on official accounts,[3][4] erasing memories,[5] tricking their opponents with staged results or false information, and creating forged historical documents to make them appear as a more kind and benevolent person than they are in reality.[6] Despite Rimuru's often selfish and nefarious tendencies, they do deeply care for their subordinates, viewing them as something like a family or collection of children. Due to the memories of Satoru Mikami and Shizu, Rimuru has access to a robust set of human ethics to rely on when their more cruel monster self isn't up to the task.
Rimuru's goal in life is to create a convenient world where they can kick back and lazily enjoy entertainment similar in quality to what is found on Earth.
