A hero that defeat Maou was being hunted, shunned, no where to go he decided to apply job at Maou castle
Meanwhile Maou and co itself is a peaceful demon that want to improve their environment using stone that available at town which is why they look antagonist
The story itself about hero helping the demon to become better 'person'.
Ok that's some pretty hefty foreshadowing of Merness turning out to be a girl. Honestly, though, I think I'd prefer it if he is actually a guy and just goes gay for the MC. That tends to be funnier
I have read this series web novel and I can say that this series have a heavy plot in it.
The actual reason why hero joined the Maou army is because to evaluate the demon king whether they have the requirement to stop him. Leo is actually one of the 12 bio weapon created by humanity to stop the invasion of the previous demon king. His main purpose is to protect the human so what he actually afraid is when the time come for human doesn't need protection anymore. Leo afraid that he might become the problem for humanity and solve it by himself to achieve his life purpose(basically a match-pump). Btw his power is copy and learn if I remember correctly(could be wrong)