Billy is a thirteen-year-old boy living in the Welsh mining town of Aberowen. He belongs to a working-class family and draws heavy inspiration from his father's unionist ideals. Deeply religious, he relies on his faith to survive the terrifying, pitch-black conditions underground.

Introduced under her maiden name, Ethel is the eighteen-year-old daughter of a Welsh coal miner. Working as the head housekeeper at the sprawling Ty Gwyn estate, she handles massive logistical challenges with ease. Her confidence and refusal to act demurely around the gentry make her stand out among the household staff.

Earl Fitzherbert is an immensely wealthy British aristocrat who owns the Aberowen coal mines and surrounding lands. He believes his social class is divinely ordained to rule over the common people. He fears social reform and works to maintain the rigid traditional hierarchy of the British Empire.

Maud is Earl Fitzherbert's confident and outspoken sister. Unlike her brother, she runs a health clinic for poor women in London and advocates for the suffragette movement. She regularly challenges the conservative views of her peers by voicing her progressive ideals.

Grigori is a young Russian factory foreman living in Saint Petersburg. He harbors deep anger toward the aristocracy following the cruel executions of his parents. Serious and responsible, he saves his money with the goal of immigrating to the United States.

Walter is a member of the German nobility stationed at the German embassy in London. Clever and modern in his thinking, he actively works to intercept intelligence and prevent a massive military conflict. He views the older, imperialistic generation as dangerous and out of touch.

Gus is a highly educated American political aide working directly for President Woodrow Wilson. Coming from a wealthy Buffalo family, he relies on intellect rather than aggression to solve problems. He closely monitors the escalating European tensions to advise the president on foreign policy.

Lev is a factory worker in Saint Petersburg and the younger brother of Grigori. Unlike his serious sibling, Lev is hedonistic, charismatic, and prone to taking dangerous shortcuts. He frequently engages in illegal schemes and relies on others to save him from the consequences.

Dai is a passionate unionist and coal miner in Aberowen. He reads progressive newspapers and passionately critiques the decadence of the aristocracy. He advocates for the rights and safety of the local coal miners, holding strict moral expectations for his children.

Bea is a Russian princess married to Earl Fitzherbert. Raised with immense wealth, she views the lower classes with open disdain and believes physical punishment is an acceptable way to manage servants. She remains largely ignorant of the simmering anger among the working class.

Katerina is a young Russian peasant woman trying to survive in the brutal environment of Petrograd. After Grigori saves her, she moves into the Peshkov brothers' apartment. She quickly finds herself caught between Grigori's quiet devotion and Lev's magnetic charm.

Otto is Walter's father and an influential conservative in the German government. He views war as a potentially useful tool for advancing German interests. He firmly expects his son to marry a German woman and uphold traditional imperial values.

Perceval is the Chairman of Celtic Minerals, the company running the Aberowen mines. He is ruthless in his pursuit of profit. He actively cuts safety measures to save money and moves quickly to evict widows of miners killed on the job.

Pinsky is a corrupt and brutal police officer operating in Saint Petersburg. He uses his authority to terrorize peasants and workers, acting as a violent enforcer for the unjust tsarist regime.

Rosa is a highly articulate American journalist. She possesses a deep understanding of the European conflict and expertly explains the strategic realities of naval blockades to Gus. Her sharp intellect frequently impresses him.

Olga is the beautiful daughter of a powerful Russian businessman. She lives a life of extreme luxury in America and is accustomed to having her desires met by her overbearing father.

Josef is a powerful Russian expatriate running multiple businesses in Buffalo. He is fiercely protective of his daughter and expects absolute obedience from his employees, easily wielding his money and influence to control those around him.

Rhys is a deputy manager at the Aberowen mine. He delights in tormenting new child workers, intentionally leaving Billy alone in the pitch-black tunnels as part of a cruel hazing ritual.

Robert is Walter's Austrian cousin. He firmly supports Austria's aggressive response to the assassination of Archduke Franz Ferdinand, reflecting the rigid alliances that pull Europe toward war.

The King of England. He represents the pinnacle of the British social hierarchy. He visits the Aberowen estate to discuss the looming threat of war with his conservative hosts.

Andrei is Bea Fitzherbert's brother and a wealthy Russian landowner. He treats the peasants working his land with extreme cruelty. He responds violently to minor infractions like grazing cattle on his property.

The President of the United States. He acts as a careful leader attempting to manage American public opinion while steering the nation's foreign policy during global conflict.

Joey is a young coal miner living in Aberowen. He works closely with Billy and the other boys in the dangerous underground tunnels.

Mildred is a working-class woman who lives in London. She rents a room from Ethel and provides her with much-needed social support in the city.

Anton is a Russian informant who works in Europe. He supplies Walter with crucial military intelligence regarding Russian troop mobilizations.

Hermia is a relative of the wealthy Fitzherbert family. She frequently attends major political and social gatherings alongside her niece and nephew.