Serenity's struggle with responsibility was really compelling. In a lot of magical girl stories, or heck, even superhero stories, the secret identity and dual life is taken for granted. One of the reasons Spider-Man was so popular back in the day was because his series actually went into the mental strain that sort of existence put on him. With Serendipity, it's the focus, and it works beautifully.

It also does a good job of showing how Serenity, at the end of the day, is still just a kid. She's still growing, doesn't fully understand yet how to handle this kind of workload, and has no one she can turn to for help. The immense strain from both her day and night life is slowly breaking her, but she loves the thrill of being a magical girl so much that she can't bear the thought of quitting. It's superb drama.