Frank is a central figure in the narrative of Argento Soma, originally created as a bio-mechanical construct designated "Extra 1." He was assembled by Dr. Noguchi from the remains of several metallic aliens that had been shot down, with the goal of creating a powerful weapon. The name Frank is a direct reference to Frankenstein's monster, reflecting his nature as a being assembled from disparate parts and brought to life through a scientific experiment. During a final maneuvering experiment at a facility known as the MORGUE, the creature was inadvertently activated by a power surge during an attack by unknown soldiers. This event caused a catastrophic explosion that destroyed the facility, seemingly killed everyone inside, and allowed Frank to escape into the wilderness.

Physically, Frank is a large, imposing humanoid entity. His most distinctive feature is his disproportionately enormous left arm, which sets him apart from the more symmetrical forms of other aliens. This arm is not just for combat; it possesses the unique ability to perform alchemy, allowing him to alter the atomic composition of materials he touches. When not engaged in battle, Frank can cover his body in a regenerative liquid metal to heal his wounds. To enhance his combat effectiveness when working with the organization FUNERAL, he was equipped with a sonic drill weapon shaped like a staff, known as the Excalibur.

Frank's personality is initially that of a confused, reactive, and largely nonverbal being, driven by instinct rather than intellect. He is feared and treated as a dangerous weapon by most of humanity. However, a fundamental aspect of his character is his deep and exclusive bond with a young girl named Harriet "Hattie" Bartholomew. For reasons that are never fully explained by science within the story, Hattie is the only person who can communicate with him telepathically. This connection was formed when Frank, wandering in the wilderness, encountered the emotionally traumatized Hattie, who had recently lost her parents. She treated him with kindness and without fear, which led to him loyally following her commands and seeking to protect her above all else. His devotion to Hattie is his primary motivation, compelling him to escape from FUNERAL's custody during a simulation exercise when Hattie became distressed by the sound of gunfire, which triggered memories of her parents' death. He fled with her to her home in Montana to keep her safe.

Within the story, Frank serves as the most powerful asset for the secret organization FUNERAL, which is dedicated to fighting the ongoing alien threat. Despite being a construct of alien parts, he is used as a frontline weapon against his own kind. This creates a significant conflict for the series' protagonist, Ryu Soma, who was once the scientist Takuto Kaneshiro. Takuto blames Frank for the death of his lover, Maki Agata, during the explosion at the MORGUE, and dedicates himself to destroying the creature. This makes Frank the central object of both tactical value and personal vengeance.

Frank's key relationships define his journey. His bond with Hattie is the most critical, acting as a moral compass for the creature. She sees him not as a monster but as a friend, calling him "Yousei-kun" or "Mr. Elf". His other major relationship is with Ryu Soma, which is purely antagonistic for most of the series, as Soma constantly seeks to destroy him. The scientists and soldiers of FUNERAL, such as Dan Simmonds and Guinevere Green, view him as a tool, while the enigmatic General Lawrence, known as Mr. X, sees him as a crucial piece in a larger strategic game.

Frank undergoes significant development as the series progresses towards its conclusion. It is ultimately revealed that Frank is not merely a random collection of alien parts, but rather a being that houses a fragment of the soul of Yuri Leonov, the first human astronaut to travel beyond the solar system and who was declared dead. The aliens attacking Earth are drawn to the planet not out of malice, but because they are following the final will of Yuri Leonov, who wished to return to his home and wife. Frank, as a vessel for part of that soul, is intrinsically linked to them. This revelation recontextualizes his entire existence, transforming him from a mindless weapon into a tragic figure. Upon learning that his human wife died long ago, Frank experiences immense grief. In the final battle, however, he achieves a sense of purpose. He merges with all the other remaining progressive aliens on Earth to form a colossal giant type. He then rockets into space at light speed to sacrifice himself, ramming into a planet-sized alien and destroying it, thereby saving the Earth.

Frank's abilities are formidable and varied. His primary innate power is alchemical transmutation via his large left arm, allowing him to restructure matter on a fundamental level. He possesses superhuman strength and durability, and his body can regenerate by secreting a liquid metal. After joining FUNERAL, he wields the Excalibur, a powerful sonic drill staff used as a melee weapon. At the climax of the series, he demonstrates the ultimate ability to absorb and merge with other aliens, achieving a giant, more powerful form capable of faster-than-light travel and delivering an impact strong enough to annihilate a celestial body-sized enemy.