The character development is a major strength. Not only Rimuru but also the supporting characters like Shion, Benimaru, etc. have their own growth and backstories. They evolve as the story progresses, which makes them more relatable and engaging. The novel also has elements of comedy, for example, Rimuru's interactions with his subordinates often lead to humorous situations, making it an enjoyable read.
Rimuru Tempest is obviously one of the most interesting characters. His transformation from a normal human to a slime and then to a powerful leader is really captivating. His unique abilities and his kind - hearted nature towards his followers make him stand out.
The SCP Foundation is purely fictional. It's a collection of imaginative and often creepy stories and characters that are made up for entertainment purposes and have no basis in reality.