Bernard Octavias, often called by the nickname Bell or Bel, is a central character in the anime. He is a seventeen-year-old boy who was raised in one of the noble families of the country of Serpenada. After graduating from a preparatory cadet academy, he enlisted in the military as an officer. His life takes an extraordinary turn when he is killed by his own men, only to be revived by a mysterious phenomenon known as the Return from the Underworld, emerging from the teeth of a giant dragon. This event brings him to the attention of the dragon dentists, the special unit tasked with caring for the mythical creature, and he is placed under the care of a young dentist named Nonoko Kishii as a trainee.

Bell's personality is defined by the stark contrast between his own worldview and that of the dragon dentists. Unlike the dentists, who have all been chosen by the dragon and have accepted visions of their own inevitable deaths, Bell is an outsider who did not undergo this test. He is a former soldier from the enemy nation, and his initial perspective is that of a pragmatic and perhaps skeptical combatant. This leads him to question the fatalistic and accepting attitude toward death that the dentists hold. Despite this, he is portrayed as relatable and sympathetic, with his confusion and resistance to his new reality making him a compelling figure. His relationship with Nonoko is central to his development; her kindness and dedication to her duty help him to gradually understand the dentists' way of life.

Bell's role in the story is that of a catalyst and a student. His very existence is considered an omen of a great disaster, causing fear among the dentists. However, Nonoko takes responsibility for him, and he begins to learn about the dangerous work of fighting cavity monsters that grow within the dragon's teeth. Through his interactions with Nonoko and the other dentists, Bell confronts profound themes of life, death, and destiny. His development culminates in a heroic act when a catastrophic event occurs; he sacrifices his life in an attempt to limit the ensuing disaster, demonstrating how much his perspective has changed from that of a simple soldier fighting for his country. As for notable abilities, beyond being a trained soldier and officer, Bell has no special powers. His primary distinction is the unique circumstance of his revival by the dragon, which is the event that sets his entire journey in motion.