This might be a controversial opinion. I agree that the main female lead is useless and air-headed, however I think it makes sense for her to be that way. As seen in the beginning, through interactions with her friends, she was sort of the 'butt of the joke' friend, the one you always tease. So, I think it makes sense for her to meek, weak and a cry-baby, especially when she all of a sudden was thrown into a scary word surrounded by messed-up people or people who were drilled to be emotionless like the main character. Also, if you are looking for an interesting female character in this manga, I think the main antagonist Kyou is the perfect example. She is not only the main antagonist and ring-leader, which you usually never seen in shounen manga or anime. But, she is also described to be literal perfection, genius, and flawless. In their childhood, you could truly see how messed up she is, wishing to be the first person to truly kill for no reason, something similar to doing something as simple as breathing air and sacrificing anyone to satiate her boredom or wishes. Truly the definition of evil. I believe we will see many great things from her in the future of this manga.
Now it sort of feels like it has something to do with the ex-girlfriend... Even though it felt like she was just an extra character. I'm so confused and curious! The plot thickens!