>>281567538
English is clearly not your first language, but the reason Megumin is so small and skinny is because she is literally malnourished. Her family eats rice soup (water with some rice floating in it) because they're dirt poor.
There are girls like that IRL, and they're usually extremely sweet and good-hearted, just like she is. She and Darkness are on equal terms because of their quality of companionship. Darkness has her kink complex (which is fine) but Megumin is actually just a good partner for life.