Rimuru Tempest

"It's simple. I want to create a world where life is as convenient as I wish it to be. A comfortable world where everyone can laugh and enjoy their lives as much as possible. That's what I truly and honestly want. Ideals without power are nonsense, and power without ideals is empty, right? I'm a greedy guy, but I don't have an interest in seeking power just for the sake of power, without the ambition to do something with it."

Rimuru Tempest (リムル゠テンペスト) is the main protagonist of That Time I Got Reincarnated as a Slime and the reincarnation of Satoru Mikami.

Slime
In their normal form, Rimuru takes the appearance of a usually spherical and sometimes amorphous mass of slightly blue and translucent ooze. Initially, Rimuru's slime form has a diameter of around 40cm, this increases to 70cm after evolving to Demon Slime and further increases to between 80-100cm after reincarnating into an Ultimate Slime.

Human
Rimuru's facial features in human form are nearly identical to those of Shizu, but the long silver 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 dragon-like 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 10cm 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 & gold shirt both woven from silk mixed with Steel-Thread. Overtop this, a thick black (blue in the anime and manga) jacket and a fur scarf both made from the pelt of the Direwolf Boss was worn.

True Demon Lord
After the harvest festival and evolution into a Demon Slime, Rimuru gains another 10cm in height, giving the appearance of a child around the age of ten to twelve. 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 15-16 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 a swords scabbard. 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 in place of the originals.

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 stark contrast to this normally composed and calculative demeanor, Rimuru is absolutely ruthless when it comes to handling people who have aroused genuine fury inside of them. They'll go as far as brutal torture and murder without 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 also takes a great deal of care with their personal image, lying on official accounts, erasing memories, and creating forged historical documents to make them appear as a more kind and benevolent person than they actually are in reality. 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. And 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.

Trivia

 * Rimuru's unofficial birthday is February 20th 2013, as it was when the first chapter of the original web novel was released.
 * A possible origin for Rimuru's name can be found in Japanese Wordplay. The 3 characters making up Rimuru (リ,ム,& ル) are all represented by the number six in Goroawase. Thus, if you use Goroawase on Rimuru, you get the 666.