Amshel Goldsmith begins his long life as a human aristocrat and research assistant to Joel Goldschmidt the First, a wealthy scholar who maintains a menagerie of exotic creatures known as the Zoo. During an expedition, they discover the mummified remains of a monstrous creature alongside two egg-like sacs. When Amshel attempts to cut one of the eggs open, his blade slips and cuts his finger, and a drop of his blood falls onto the cocoons, triggering a reaction that causes them to hatch. From these eggs emerge two seemingly human infant girls, who are later named Saya and Diva, revealed to be a previously unknown species called Chiropterans. Joel decides to raise Saya as his own daughter while keeping Diva confined as a research subject in a tower, where Amshel is responsible for bringing her food and tending to her basic needs.

In 1883, Diva escapes from her confinement and goes on a murderous rampage. She spares Amshel during this event and transforms him into her first chevalier, a higher order of Chiropteran bound to serve a queen. After Joel's death, Amshel assumes control of the research into Chiropterans and takes charge of creating subsequent chevaliers, beginning with Solomon, a distant relative of the Goldsmith family. Over the decades, he builds a powerful business conglomerate called Goldsmith Holdings and becomes the bureaucratic head of the organization known as Cinq Flèches, using its resources to fund his experiments and operations.

Amshel possesses a deeply twisted personality driven by scientific curiosity and a possessive obsession with Diva. He refers to her as his little treasure and confesses that he wishes Saya had never freed her from the tower, because then Diva would have remained his and his alone. He views his devotion to her as the ultimate form of love, yet he simultaneously treats her as an experimental subject for his research into Chiropteran biology and reproduction. His ambitions are purely self-serving, and he is willing to perform any action, no matter how cruel, to further his own goals. Despite referring to his fellow chevaliers as his brothers, he shows no genuine concern for them, remaining unaffected by Karl's death and ordering the execution of Solomon when he develops feelings for Saya.

As the leader of Diva's chevaliers, Amshel serves as the primary antagonist working against Saya and the Red Shield organization. After Saya awakens from her thirty-year sleep, Amshel initially attempts to persuade her to join their side. In Russia, he kills a Red Shield contact named Liza and assumes her identity to infiltrate a train journey to Ekaterinburg. During this encounter, he reveals to Saya her true nature as a Chiropteran and that her sword forces her to kill her own kind. When she remains determined to fight and destroy Diva, he decides she must be exterminated. He is also responsible for creating the Schiff, an artificial race of half-human, half-Chiropteran beings, raising them as child soldiers and discarding them when they outlive their usefulness.

His relationship with Diva is one of possessive control rather than genuine affection. He carefully shaped her development from childhood, fostering her violent tendencies to suit his purposes. He is the only person Diva truly listens to, and he uses this influence to manipulate her for his research. At one point, he confesses to having sexual relations with her to determine whether she could bear children, further demonstrating his view of her as little more than a specimen. By the end of the series, his interest in Diva wanes entirely, and he shifts his focus to her newborn twin daughters, intending to raise them as he did her. His relationship with Solomon is primarily transactional; though they share blood ties, he uses his relative as an assassin and pawn. Among his fellow chevaliers, Nathan is the only one whose movements Amshel cannot control or predict, leading to tension between them.

Throughout the story, Amshel demonstrates himself to be Diva's first and most powerful chevalier. He possesses superhuman strength and speed far exceeding that of average chevaliers, having easily caught and crushed Saya's arm when she attacked him and intercepted a dagger thrown by Solomon to protect Diva. He exhibits telepathic communication with other chevaliers and displays telekinetic abilities, such as breaking objects with a gesture. One of his most notable powers is shapeshifting, an ability only otherwise demonstrated by Diva herself; he impersonates several characters throughout the series, including the Red Shield agent Liza and even Diva herself in the final episodes. In his full Chiropteran transformation, which he finally reveals in the penultimate episode, he takes on a large, demonic, winged form with six limbs similar to Hagi, the only other chevalier with such a configuration. In this state, he can emit powerful blasts of energy from his voice and survive devastating injuries, including being impaled on the Chrysler Building and struck by lightning.

In the final confrontation, Amshel shapeshifts to resemble Diva in an attempt to delay Saya while the real Diva completes her song. He engages in an aerial battle with Hagi over New York City, where he is impaled on the Chrysler Building and struck by lightning. He reappears in a half-human, half-Chiropteran form, still intent on capturing Diva's newborn daughters. Hagi finally destroys him by stabbing him with Saya's blood-covered sword, causing him to crystallize moments before the opera house collapses.