Rimuru Tempest

Previous Life
Satoru Mikami, he is a 37-year-old human virgin who died protecting a colleague. Reincarnated as a slime in another world, he becomes a friend of Veldora Tempest, one of the Four True Dragons, and slowly accumulated power through his acquisition of Skills and subordinates. Rimuru is the founder and King of Monster Country Tempest in the Jura Forest.

Rimuru eventually becomes one of the Demon Lords, and eventually, a Great Demon Lord. He acquired a vast amount of power and becomes the most powerful existence in the universe. His subordinates are numerous; Twelve of them are Awakened Demon Lords, who are equal to or more powerful than some of the members of the Octagram.

Slime
Rimuru initially appears as a white-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.

Mimicry: Human
In this form, Rimuru possesses 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-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.

Since eating the Orc Disaster, his human form has grown a bit. He now assumes the form of a pre-teen with feminine features. In this form, he is around 150 centimeter(5FT) in height. In addition, he has obtained a devil transformation.

Upon becoming a True Demon Lord and changing his race to Demon Slime, Rimuru has been depicted wearing a black Kimono. 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 that of a 16 to 17-year old. This form is described by Velgrynd 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 is able to mimic any age.