@Doenyon
It's not that hard to understand. Admittedly there are still some parts that are unclear, there are a few hints dropping thorough chapter 2 to 4
1) Definitely something happened to Dorothy. A) When she was resurrected, she asked if Faris was a human, and since she isn't, Dorothy is happy to join MC. B) She feels pity for the people being eaten by a vampire, and she said she knows it too well. That tells us that she was killed by a vampire. Her backstory might be revealed later on so calm down and let's see how it unfolds. This is only her first appearance.
2) Faris's power is questionable. For now, she is assigned to be the last section before Aria. The reason being, she is the last resort, plus not to mention she isn't a human. We don't know who she is what yet.
3) The whole point of chapter 3 is they are lacking manpower. Ron already explained but also, Dorothy revealed something that Aria is the genuine Great Demon Lord, which tells us there might be other (minor/small) demon lords as well.
If you recall in chapter 2, the ratio of demon to human is 7:3 (or 8:2, I can't remember). So it's reasonable to assume that the demons are fighting among themselves as well.