One of the main themes is growth. The characters in the novels often experience significant personal growth, learning from their experiences and mistakes. Another theme is friendship. The relationships between the characters play a crucial role in the story development.
The Beginning After the End manga has indeed finished. It provided a satisfying conclusion to the characters' arcs and the overall narrative. Maybe there'll be spin-offs or related projects in the future, but for now, the main story is complete.
Caera is another notable character. Her interactions with Arthur are interesting as they come from different backgrounds. Her character brings in new elements to the story, especially in terms of different cultures and power systems within the world of the novels.