Yu is a young male apprentice voluntarily assisting engineer Vulcan Joseph. He stands 142 cm (4'8") tall, weighs 39 kg (86 lbs), and sports long black hair parted in the middle, featuring a braided ponytail secured with a white ribbon. A single strand of hair often sticks up from the center of his head, and he wears two crossed adhesive plasters on his forehead; his eyes are light gray in the anime. His typical outfit includes a black jacket with light-colored buttons and a cross insignia on the collar, worn over a blue sleeveless shirt featuring three white horizontal stripes with white outlines, paired with black trousers and shoes. He is frequently seen carrying a large backpack. He exhibits a kind and responsible personality, showing keen interest in mechanics and diligently scavenging for materials to support Vulcan's unsupplied workshop, driven by admiration for Vulcan's technical skills. Yu demonstrates courage, such as defending Shinra Kusakabe during Company 5's apprehension despite being physically pushed down. In the Vulcan's Workshop arc, Yu meets Shinra, Arthur Boyle, and Iris outside the workshop, recognizes Shinra as his past rescuer, and facilitates their entry. He later explains Vulcan's distrust of Haijima Industries, revealing the simultaneous Infernal transformation of Vulcan's father and grandfather – a tragedy Yu suspects was orchestrated by Giovanni, Vulcan's grandfather's former apprentice who joined Haijima. Yu actively advocates for Vulcan's independence, warning Haijima would stifle Vulcan's inventive freedom. When Giovanni attacks the workshop, Yu is strangled to force Vulcan's surrender, sustaining injuries requiring Iris's medical treatment. After recovery, he thanks Company 8 for their intervention. Yu later joins the Arthur's Adventure arc, accompanying Arthur, Lisa Isaribi, and Vulcan into the Netherworld after Vulcan's promise to find orichalcum for Excalibur spirals into an unexpected journey. His name "Yū" is written in katakana (ユウ). Potential kanji interpretations are 勇 (bravery/courage), 優 (gentleness/excellence), or 悠 (permanence), reflecting aspects of his character. His shoe size is 24 cm and his eyesight measures 1.5 (20/12.5).