The plot involves Rimuru's journey from being a rather weak (initially) slime to becoming a very powerful being. He meets many interesting characters. For example, there are the goblins he befriends early on. Rimuru helps them improve their living conditions. As the story progresses, he also has to deal with the threat of other more powerful monsters and even some human kingdoms that view his growing power with suspicion. In general, it's a story full of adventure, growth, and the forging of relationships in a magical and dangerous world.
Since I don't have detailed knowledge of 'nikki benz pump fiction', I can't say for sure who Nikki Benz is in it. She could be a protagonist with unique qualities and a backstory.
I'm not sure who Nikki Yovino is just based on this title. She could be the author of the story, in which case she might be a writer or storyteller. She might be writing from her own experiences or from her imagination.
The main character is Rimuru Tempest. He is a slime who has unique powers and a great deal of charisma. Another important character is Veldora, the storm dragon who befriends Rimuru. There is also Shizu, a human who has an impact on Rimuru's early development. And then there are characters like Benimaru, a powerful ogre who becomes one of Rimuru's loyal followers.