Francis "Frank" Cook was born in Minneapolis in 1982, becoming a country boy who dreams of seeing the world. He excels in school, but the violent death of his parents turns his life upside down. Little Frank has no one to turn to anymore and, to fulfill his desire to see the world, he enters the Marines' officer school. He quickly manages to pass every exam and becomes a lieutenant.

Thanks also to his intelligence and his preparation, he soon distinguishes himself and is inserted at a very young age in the Marines' Special Operations unit. Inside, Frank participates in missions of great strategic importance in South America, against the Colombian drug cartels.

In the last of those operations, Frank's Marine team, cooperating with Delta Force, is annihilated by a Cartel ambush. The young lieutenant is the only one to survive the ambush and the subsequent escape into the Amazon forest: his exploits are immediately noticed by the UNSPF military, who consider him an excellent recruit for the new space organization.

Frank is curious and immediately accepts this incredible opportunity. After a short training on the lunar base Selenica, he is integrated into the crew of the flagship of flagships, the ISS Borea, stationed in the orbit of Neptune.

Irina Ivanova is the main female character in the Sky Domain saga.

Daughter of the Soviet Union Commissar Gregori Ivanov, she already as a child establishes a special relationship with her father, with whom she often goes hunting in the Siberian steppe. It is he who teaches her, at a very young age, to use a rifle and a pistol.

Commissar of the NKVD, Gregori, in the last years of the Soviet Union, conducts uncomfortable investigations, which see him follow a secret US special team. His uncomfortable investigations are the cause of his death, which occurred under mysterious circumstances.

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Irina has always wanted to find the reason for the death of her father, to whom she is so attached. His last words suggest her to "find the answer in the stars", and so Irina does everything to be able to join the UNSPF, which she succeeds in doing when she distinguishes herself in a rescue operation in Kazakhstan, when she is still part of the Spetnaz, the Russian special forces.

Pluto is not the first mission Irina has been assigned to in the UNSPF: before that, she expertly manages a failure on Mars, which earns her confirmation of her previous rank, Captain.

Irina's character is always rebellious: nonconformist, against the rules, vulgar and violent, Irina is a strong woman who opposes the authorities and who perhaps never wanted to be a soldier, perhaps she really wanted a different future. Yet, being a soldier is incredibly good for her and makes her one of the main heroines of the Earth.

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Alexandru Brinzaru is the most important scientist in the Sky Domain saga.

A graduate of MIT in Boston in materials engineering, he was contacted by UNSPF after several research articles on ferrofluids and their modeling in space environments.

Integrated into the crew of the flagship ISS Borea, the application of his knowledge to bionium allows him to quickly become the head of a research center on this material.

Brinzaru's knowledge is also remarkable in other fields, such as celestial mechanics, astronomical geography and astronomy in general: it will be precisely this knowledge that will make him a key element in the war between humans and Dullahan.

He is a haughty person, a haughtiness that he will lose in the development of his friendship with Frank. Despite his constant desire to discover new things, he is afraid of death and often stops Frank's crazy plans.

Matthew has a dark past: a policeman in the suburbs of Birmingham, England, he suddenly decides to join the British Army. Employed in the war in Iraq, Matthew immediately shows his value, fighting with extreme courage, almost not considering his life. It seems like he wants to find a pretext to die.

He comes very close to achieving his goal: during an ambush on his convoy, despite once again demonstrating incredible courage, he is wounded and abandoned in the desert.

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Near death, it is perhaps there that Matthew reflects on his own existence, on his purpose in life. When everything seems lost, however, he finds salvation and the strength to move forward.

He counterattacks the enemy and, in a new suicide mission, breaks into the headquarters of the Iraqi terrorist, arresting him.

The move is immediately noticed by the high command and he is offered to join the UNSPF, where his shady character will clash with his destiny of glory.

Thomas “Tom” Oliver Marshall is a curious character, because starting out as a scientist, he becomes one of the most powerful military men in humanity.

In the immediate post-war period, Tom graduated from MIT in Boston with full marks in Biomedical Engineering, with an extraordinary thesis on the advanced symbiosis of living systems.

Tom's profile reaches the ears of NASA scientists who are first studying bionium: they immediately involve Tom in that research, of which he soon becomes the protagonist. He is transferred to the Moon, where he formally begins to be part of the United States space army, the NASF.

But young Tom is not the classic book-shy nerd: Tom is dynamic, enterprising and extroverted. He plays a thousand sports and despite this he is one of the most brilliant minds on the planet. For this reason he has no trouble climbing the NASF hierarchy, also acquiring important leadership skills. His scientific career actually takes a back seat to his military career, especially when the conflict between the United States and the Soviet Union breaks out.

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It is in fact by leading various special teams that he manages to climb the ranks and, when he manages to put an end to the entire war by organizing an armistice with the Soviet forces, he is appointed Supreme General of the new force that will have to restore peace in the Solar System, the UNSPF.

The figure of Michael "Mike" Sampson is extremely dark and poorly defined.

Born in the United States after World War II, he fought in Vietnam in the First Cavalry Division, immediately showing great military skills that made him perfect to be in command of special teams of the United States Space Army, to oppose the Soviet Union in the war that developed in space. In fact, thanks to his command, risky rescues and daring operations succeeded.

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Little is known about what happens to Michael, except that many of those daring operations are ordered by his new commander, a man more of science than of weapons: Thomas Marshall. Despite his initial mistrust, Michael develops a strong friendship with his superior.

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However, after the war against the Soviet Union ends, he disappears, there is no trace of him.