Paddy is an unhoused youth living on the streets of Belfast who survives by pickpocketing. Scrawny and underfed, he relies on his quick wits and physical endurance, having once walked 67 miles to escape an abusive stepfather. His street smarts are put to the ultimate test when he accidentally stows away aboard the Titanic while fleeing violent criminals.

Juliana is the 15-year-old daughter of an earl, traveling first-class to New York. Having grown up strictly within high-society circles, she initially views servants as functional objects rather than people. Her voyage exposes her to different perspectives that challenge her rigid understanding of the world.

Sophie is the 14-year-old daughter of an American suffragist, deported back to the United States via first-class passage on the Titanic. Frustrated by her mother's tendency to prioritize activism over family time, Sophie has nevertheless internalized a strong sense of justice. She bridges the gap between the different social classes represented by the other youths.

Alfie is a 15-year-old who lies about his age to secure a job as a first-class junior steward aboard the Titanic. Unlike the other youths, he enjoys a healthy and supportive relationship with his father, giving him a sense of emotional stability. Oblivious to the strict social rules of the upper class, he unites the diverse group of children through a shared mystery in the cargo hold.

The gangsters are notorious brothers who run organized crime in Belfast. After Paddy and Danny accidentally pickpocket them, the violent thugs pursue Paddy onto the Titanic to exact their revenge, terrorizing him throughout the lower decks.

Danny is Paddy's best friend and partner in surviving the streets of Belfast. He possesses a brilliant mind for engineering and problem-solving, famously deducing a critical flaw in the Titanic's allegedly perfect design. An encounter with local criminals separates the two boys before the ship sails.

Thomas Andrews is the proud designer of the Titanic. He interacts with Danny in Belfast, explaining the ship's compartmentalized hull design that makes it supposedly immune to sinking. He acts with the absolute confidence of the ship's creators, believing his vessel to be a perfect triumph of modern engineering.

Amelia is a prominent American suffragist and Sophie's mother. She frequently argues with the first-class male passengers about women's rights, prioritizing her political causes and activism over spending quiet time with her daughter.

The Earl of Glamford is Juliana's father, traveling on the Titanic to meet with business associates in New York. He spends most of his time aboard the ship gambling and drinking, largely leaving his daughter to manage her own time during the voyage.

Alfie's father is a hardworking man who shovels coal in the boiler rooms of the Titanic. Unlike the wealthy parents of the upper-class children, he provides a steady and reliable source of love and support for his son.

The captain of the Titanic is a capable authority figure who refuses to let thugs bully him or dictate his actions. However, his strong faith in the ship's advanced technology leads to a dangerous sense of overconfidence regarding natural threats in the ocean.