The villains are also important main characters in a way. They could be other spirit detectives who have gone rogue or some sort of dark entities in human form. Their actions and motives drive the plot forward as the twins try to stop them.
One possible plot twist could be that the twins are actually not related by blood but are part of some mysterious experiment. This would turn their whole identity and purpose as detectives on its head.
Since it's a detective novel from 1929, there could be a lead detective who is smart and observant. There might also be his sidekick, who provides some comical relief or different insights. And there could be the villains, who are trying to outwit the detectives and carry out their evil plans.
I don't know for sure as I haven't read this 'twin novel'. But they could be the twins themselves, and perhaps their parents or friends who play important roles in their lives.
The main characters would be the dead boys who are detectives. They might have different personalities. One could be more adventurous and eager to explore the afterlife, while the other might be more cautious and analytical. There could also be some supporting characters, such as spirits they meet along the way who either help or hinder their investigations.
I don't know for sure as I haven't read it, but perhaps there are boys named Tom, Dick, and Harry.
I don't know for sure. But there could be a tough - as - nails detective who is the leader of the group. Maybe a young and inexperienced detective who is learning the ropes.
Well, in some popular 'three detectives novels', the main characters could include a detective who is a bit of a loner but has a brilliant mind for solving puzzles. Another might be a detective who has a family connection to the criminal underworld, which gives them an inside perspective on certain cases. And the third could be a detective who has just joined the force and is eager to prove themselves, bringing a fresh perspective to the investigations.
I don't know as I haven't read this 'twin stars novel'. There could be all sorts of main characters. Maybe it has a male and a female lead who are the 'twin stars' in a metaphorical sense.
As I'm not familiar with this 'twin sword novel', it could have a main character who is a skilled swordsman or swordswoman. There might also be a mentor - like figure who guides the protagonist in their journey related to the twin swords. And there could be an antagonist who wants to get the twin swords for their own evil purposes.
Since it's about dead boy detectives, the main characters are likely those dead boys themselves. But without reading it, I can't give you specific names.