Perhaps there is a character who is the epitome of the 'dark heart'. This character's actions and decisions drive the plot forward. And then there could be a love interest who either tries to save the character with the 'dark heart' or is affected by it. There may also be a sidekick character who provides some comic relief or different perspective in the face of all the 'dark heart' drama.
I'm not sure specifically as we don't have more details about this 'The Dark Heart Novel'. But generally, there could be a protagonist who is on a journey to understand or overcome his or her 'dark heart'. There might also be a mentor - like figure who helps the protagonist in this exploration.
In 'The Dark Heart Novel', there may be an antagonist who represents the purest form of the 'dark heart' forces. This antagonist would oppose the protagonist at every turn. There could also be a group of secondary characters, like a family or a community, whose reactions to the 'dark heart' elements within the main characters help to shape the overall story.
Marlow is a very important character. He is the narrator of most of the story and his journey into the heart of Africa forms the core of the narrative. Kurtz is also crucial. He is a man who has gone deep into the Congo and has been corrupted by the power and the environment there. The Manager is another important character. He is in charge at the Company's station in the Congo and is often in conflict with Marlow and represents the more bureaucratic and self - interested side of the European presence in Africa.
Scobie is an important character. He is a police officer and his actions and decisions drive much of the plot.
Reuben Butler is an important male character. He is a love interest for Jeanie and his own moral and intellectual qualities play a role in the story. He also represents a certain type of man in that society, one who is trying to make his way through education and good deeds.
Lyra Belacqua is a central character. She has a daemon named Pantalaimon, and her adventures in different worlds are what drive the plot forward. Will Parry is another important one. He has a special knife that can cut through different dimensions. And Mrs. Coulter is an important character too. She is a complex figure, sometimes seeming to be an antagonist but also having her own motives and backstory that make her more than just a simple villain.
Mick Kelly is also significant. She is a young girl with big dreams and a passion for music. Her relationship with Singer is quite complex.
Marlow is a very important character. He is the narrator who takes us on the journey into the heart of darkness. Kurtz is also crucial. He is a man who has become corrupted by the power and isolation in the Congo. Then there are the native Africans, though they are often not given as much individual depth, they play a significant role in representing the 'other' in the context of the colonial story.
Guilliman is an important character. He is a key figure in the Imperium's struggle against Chaos.
In 'Beyond the Dark Portal novel', Thrall is a crucial character among the orcs. He is a strong leader with a vision for his people. On the human side, there are characters like Alleria Windrunner. She is a skilled ranger and her actions are significant in the battles and events that unfold. Also, Khadgar is another important character. He is a powerful mage who has knowledge and abilities that are vital in understanding the Dark Portal and what lies beyond it.
Acheron himself is surely an important character. He's likely to be at the center of the story, driving the plot forward. There may be some sidekick characters who help him in his quests. And probably an antagonist, someone who opposes Acheron's goals.
The most important character is Marlow. He's the one who undertakes the journey up the Congo River and has all the encounters that form the basis of the story. Kurtz is another significant character. He has a sort of magnetic pull on Marlow and represents the extreme of what can happen to a man in that environment. Additionally, the Manager of the trading post is an important character as he is part of the power structure in the Congo and his actions and motives contribute to the overall atmosphere of the story.