Yû is a young male standing 142 cm tall and weighing 39 kg, with long black hair parted in the middle and a braided ponytail secured by a white ribbon. A distinctive feature is the two crossed plasters on his forehead, complemented by light gray eyes. His typical attire includes a black jacket featuring light buttons and cross insignia on the collar, worn over a blue sleeveless shirt with three white horizontal stripes. He wears black trousers and black shoes, and consistently carries a large rucksack. He possesses a kind, responsible, and courageous personality, demonstrated when he defended Shinra Kusakabe against Special Fire Force Company 5 despite being overpowered. His primary motivation is a deep interest in mechanics and machinery, leading him to become an apprentice to the engineer Vulcan. Since Vulcan lacks official support, Yû scavenges for materials to facilitate their work. His involvement begins when Shinra rescues him from an Infernal, establishing his respect for Shinra, whom he identifies as a hero. During the "Vulcan's Workshop" arc, he facilitates the meeting between Shinra, Arthur Boyle, Iris, and Vulcan by allowing them entry against Vulcan's reluctance. He reveals critical backstory: Vulcan's family history, the suspicious simultaneous deaths of Vulcan's father and grandfather (who became Infernals), and the antagonistic role of Dr. Giovanni—Vulcan's grandfather's former apprentice now aligned with Haijima Industries. Yû expresses concern that Haijima Industries seeks to exploit Vulcan's inventions while suppressing his creative freedom. Yû faces direct antagonism when Giovanni attacks the workshop, strangling him to force Vulcan's surrender of the Key to Amaterasu. This results in injuries treated by Iris. Despite this trauma, he returns to Vulcan's workshop after recovery, reaffirming his commitment. Later, in the "Arthur's Adventure" arc, he participates in a journey into the dangerous subterranean Nether alongside Arthur, Lisa Isaribi, and Vulcan to locate orichalcum for Arthur's sword, leading to an encounter with Arthur's parents and the acquisition of pre-Calamity materials. Across these events, Yû evolves from a supportive apprentice into an active field mission participant, with his consistent loyalty to Vulcan driving actions that advance both Vulcan's objectives and their allies' broader goals.