I'm not entirely sure as I haven't read it specifically. But generally, it might be about the adventures of Emil and some detective - like elements. Maybe Emil gets involved in solving mysteries or has some exciting encounters in a city or town setting.
Since I don't have access to the 'emil und die detektive graphic novel', we can only guess. Obviously, Emil is probably a main character. There may be a group of kids or adults who join him on his detective adventures. There could be a wise old man or woman who gives them clues or advice. And there might be a villain or two who are causing trouble in the story. It's also possible that there are some side characters like shopkeepers or policemen who interact with Emil and the other main characters and help move the story along.
The 'Walter Boys story' might be about a family named Walter. It could involve their daily lives, relationships, and the various adventures or challenges they face. Without more specific information, it's hard to say exactly, but it likely centers around the Walter boys in some way.
The Walter boys themselves are surely important characters. There are likely to be a number of them, each with their own personalities. And of course, the main character who has to live with them is important. Maybe there's a parental figure in the Walter family as well, like a mother or father who plays a role in guiding the family and interacting with the new addition to the household.
One of the main themes could be family. The story might explore the dynamics within the Walter family and how the main character fits into it. Another theme could be growing up. As the story progresses, characters may face challenges and experiences that help them mature. Also, friendship could be a theme, with the relationships between the different characters being developed and tested throughout the story.