That Time I Got Reincarnated As A Slime
Rimuru Tempest, formerly a simple Salaryman named Satoru Mikami from Japan until he was murdered, is a Slime and ruler of the monster nation of Tempest in the Great Jura Forest, as well as one of the Demon Lords of Octagram.
As a Slime his body is made entirely of light blue liquid, able to change his size and shape at will and even change his form entirely in to other creatures including taking a human form. His human form was gained from Shizue Izawa, another otherworlder from Japan who's dying wish was that he eat her, such his human form is her spitting image with the exception of blue hair and golden yellow eyes which more often than not results in him being mistaken for a girl. Being a Slime he has no actual physical gender and any form he assumes take on an androgynous state, he continues to identify as male however since he was originally a man in his previous life.
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. He only wishes to create a place where all people can live happily regardless of race and where he can live a peaceful life, using his knowledge from his previous life to help his friends recreate a number of the modern amenities he was so fond of. While generally a kind and caring person he can still be cruel and merciless when provoked, especially to those who harm the people he cares for. This was first shown from the Kingdom of Falmuth's invasion of Tempest that killed many of his people, when in retaliation he singlehandedly wiped out the enemy nations army and consumed their souls to fuel his evolution in to a Demon Lord.
A constant companion to him has been a particular skill he obtained the moment he awoke as a Slime. It had evolved from Extra Skill Sage during his death, as he thought that since he was approaching his forties as a virgin, he must be something much greater than a mere "wizard". First called the Unique Skill Great Sage, it was a Conceptual Intelligence with no real will of its own, caring for nothing but the benefit of its master and even going so far as to hide things from Rimuru if it deems it beneficial to him in the long run. It helped Rimuru to understand things about the new world around him and new skills that he obtained. Upon his evolution to a Demon Lord, Great Sage also evolved to Ultimate Skill Wisdom King: Raphael as a result of the skills own desire to evolve for the sake of its master, having become more self aware and beginning to develop its own ego. Later, Raphael would finally become a completely self aware entity when finally given a name by Rimuru, becoming the Ultimate Skill Core Mana: Ciel, a sentient intelligence core that exists within Rimuru's consciousness. She is cunning and mischievous but at the same time very kind towards her allies, especially to those who hold good intentions towards Rimuru. Oftentimes, due to the fact that she knows how her master thinks, she would hide certain information from him which would cause him to be surprised and shocked. This is of course still out of good intentions as Ciel always wants the betterment of her master, and she realizes that even if Rimuru is kindhearted and smart, he at times is a little too forgiving, naรฏve, and may accidentally cause a blunder. Thus she searches for the most optimum solution without causing Rimuru displeasure.
By the end of the story Rimuru's power becomes quite literally limitless. From first being able to copy the abilities of consumed creatures when he was first reincarnated, Rimuru gains more and more skills over time with many of his abilities evolving in to greater power. With the combination of Information King: Akashic Records, Void God: Azathoth and Harvest Lord Shub Niggurath, Rimuru is able to use any ability that exists, gift skills to others, create anything from objects, creatures, entirely new skills and even entire worlds, stop and travel through time and even between worlds.
Alternative: Forsaken Future
Instead of successfully escaping from his battle with Hinata Sakaguchi on the outskirts of the Kingdom of Ingrassia, Rimuru was defeated and almost completely destroyed by the Otherworlder. He was unable to completely regenerate until three years after his defeat, immediately returning to Tempest only to find that it had been completely subjugated by the Kingdom of Falmuth. Those monsters that hadn't been killed by the humans had been enslaved to do their bidding. Being found by Souei he guided to the territory of the Lizardmen, the only safe refuge for those monsters that had happened to be away from Tempest during the invasion.
Rimuru learned that other than Souei, Soka, Gabiru, a number of High Orcs and Hobgoblines, messengers and merchants that had been sent to Eurazania and the three Dryad sisters led by Treyni, anyone else who was considered a monster had been killed by the humans. Broken by the realization he attracted the attention of a certain demon and spurred on by his advice and confirmation from Great Sage that it could be done, Rimuru used the anger of the souls of those he had lost to fuel his evolution and became a True Demon Lord.
Rimuru's evolution to a Demon Lord changed both him and those who still followed him, bother physically and mentally, along with a drastic increase in all their abilities. His own physical changed were notable from the horns now protruding from his head, starting as white from the base and changing to black at the tips, and a red streak running down his cheek from his right eye. He accepted the demon, now named Diablo, in to his ranks and together the survivors of Tempest retook their home under the new Demon Lord
Establishing his new position, Rimuru granted blessings to those people and nations that accepted his rule while utterly crushing any who resisted. After having built a peaceful nation that welcomed all only for it to be take and those he loved slaughtered he no longer cared to show mercy to any who crossed him. Those who had survived Falmuth's invasion three years before continued to serve him loyally, along with the black wolf Ranga and the now restored Storm Dragon Veldora at his side. Rimuru grew to be known at the most evil and powerful among all Demon Lords, and would commit any atrocities he deemed necessary to get what he wants.