The major characters in the novel are:

1. Hercule Poirot - A retired Belgian detective who is known for his sharp mind and deduction skills. He is the protagonist of the story and is called in to help solve the ABC murders.
2. Captain Arthur Hastings - Poirot's close friend and a former army officer who serves as the narrator of the story. He assists Poirot in his investigation of the murders.
3. Inspector James Japp - A Scotland Yard detective who works closely with Poirot in the investigation.
4. Alexander Bonaparte Cust - A timid and mild-mannered man who is the main suspect in the murders.
5. Megan Barnard - A young woman who is engaged to Cust and provides an alibi for him during the murders.

The minor characters in the novel include:

1. Franklin Clarke - A wealthy businessman who is the first victim of the ABC killer.
2. Betty Barnard - A young woman who is the second victim of the ABC killer.
3. Sir Carmichael Clarke - Franklin Clarke's brother who becomes a suspect in the murders.
4. Thora Grey - A former nanny who is briefly considered a suspect in the murders.
5. Donald Fraser - A friend of Betty Barnard who is briefly considered a suspect in the murders.
6. Lady Hermione Lytton Gore - A wealthy woman who is briefly considered a suspect in the murders.