Initially appearing as a transparent blue bodied Slime, Rimuru is at first disappointed and embarrassed by his new “unremarkable physique” being weak, blind, deaf and without a sense of taste or the ability to speak. He describes his body as “kinda cute but not him”. Rimuru’s slime body is in the shape of a water droplet. As he acclimates to his new goopy existence he starts to grow more fond and loving of his body starting to refer to his body shape as “beautiful and magnificent” and upon obtaining the Human Form he sacrificed so much for he almost immediately misses his Slime body transforming back into Slime form and saying “Yeah, this feels much more right”. On his Slime Body Rimuru has two indentations for "eyes" and "facial features" however he cannot initially see with them, instead he relies on Magic Sense for his sight (a skill that allows those who are even blind or deaf to see). He uses these indentations to make various expressions of surprise, shock, anger, excitement and so much more. After obtaining the Sound Waves of the Giant Bat he devoured, Rimuru can now talk out loud, with the ability to create a mouth for himself to talk with and he sometimes does but he prefers to talk through his thoughts and mind the majority of the time. Rimuru uses his Slime body to store various objects, monsters and a lot of water with liquid occasionally being seen inside of him. Rimuru explains that "being a Slime by definition he never gets dirty" and is always smooth, pure and clean as he absorbs any dirt, due to this Slimes are often used for cleaning.
Human Mimicry
Upon assuming Human Form, Rimuru is quite a different Beast, literally. Rimuru's human camouflage may look normal on the outside however it is anything but. He has no internal or external organs so instead his insides are filled up completely with the mass of his Slime Body meaning his height is in proportion to the size he can stretch his Slime Form out to. With a short stature, slight build and an androgynous body devoid of any sexual characteristics whatsoever he has an unusual mystique about his appearance. He has a dazzling complexion and long flowing light silver blue hair that eventually grows all the way down to his waist, perfectly complimenting his pure and fetching figure. He finds his Human Body much easier to move in than his usual Slime Body and states that "it feels great to have an actual human body" flexing his new hands in disbelief however he laments that his hearing and sight are still much better when in Slime Form due to Magic Sense as in Human Form he is now only able to see out of a pair of eyes whereas in Slime Form he could see 360 degrees all around him. Rimuru goes through three major evolutions throughout the series: Slime, Demon Slime and Ultimate Slime. I will outline their appearances below.
Slime ("Viscous Creature")
Slime Form
20-30cm
Human Form
120-139cm
Ah, those young, innocent days they didn't last long, did they? Rimuru's "Childlike" form as he describes it. This form does grow slightly taller from 120-139cm to a "Pre-Teen form" with longer hair in addition after Rimuru devours the Orc Lord but apart from that there are no major upgrades to it. Both a material and spiritual being. He is often mistaken for a young girl at this stage and called a "Lil' Lady" which he states that he finds "needlessly cruel".
Demon Slime ("Demonic Viscous Spiritual Body")
Slime Form
70cm
Human Form
149cm
Rimuru's mid-stage evolution could sort of be described as the "Angsty Teenager" of the three at the appearance of around 14-15 years old. Now taller, with waist length hair, sharper fingernails and garbed in gothic gloom after his bitter battle with the Kingdom Of Falmuth, this Rimuru is still kind but he has a darker edge to him that he wants you to know about. There have been no other Slimes capable of evolving to this point, so Rimuru is the absolute exception. He is now capable of switching between his material and spiritual body at will making him impervious to even more attacks. His appearance is described as having become more refined with him being capable of bewitching any onlookers regardless of gender.
Ultimate Slime ("Viscous Draconic Demon God")
Slime Form
80cm
Human Form
159-160cm
The final evolution Rimuru goes through, and unsurprisingly his most grand. In the most basic of words, his fully grown 16-17 year old "Young Adult" form but able to mimic any age he likes. Becoming the Highest Tier Divine Spirit with the blessing "Rimuru's Compassion", he is unstoppable after this evolution being able to exist in multiple places simultaneously. Reaching the pinnacle of his height he is 5ft3 with his hair becoming tinged with purple, growing to its full length reaching down past his waist. As an Ultimate Slime, Rimuru has finally left his "emo-teen" phase behind him and has matured. He is described as "dangerously cute" above all else but "clad in a divine aura and presence his beauty was sublime" and he is mistaken for a beautiful woman by those who meet him for the first time. Now wearing mostly white for his everyday outfit, with black clothing for combat, he has a truly angelic appearance.