The character of Ratchett is crucial. He is the victim, but as the story unfolds, we learn that he is not as innocent as he might first appear. His past actions seem to have led to his untimely death on the train. All the characters' backstories and relationships play a vital role in making the plot of 'Murder on the Orient Express' so engaging.
Colonel Arbuthnot is also among the main characters. He is a military man, and his relationship with some of the other passengers and his actions during the investigation play an important role in uncovering the truth about the murder on the Orient Express.
I don't know the specific main characters as I haven't read it. But usually, in such novels, there would be a detective - type character to solve any mysteries, a wealthy passenger who might be involved in some sort of trouble, and a train conductor who has knowledge about the train and its passengers.
Since I haven't had the chance to read the 'orient gate graphic novel', I can't say for sure. However, it's common for graphic novels to have a protagonist who is on a journey. Maybe there's a young person who discovers the orient gate and becomes the central character. There might also be a mentor figure or an antagonist who tries to stop the main character from achieving their goals related to the gate.
The main character is Hercule Poirot, the brilliant detective. And there are the various passengers on the Orient Express, like Ratchett, who is the murder victim.
I think Mary Debenham is quite memorable. She is a calm and composed young woman, and as the story progresses, we find out she has her own connection to the case which makes her an important character.