Yū

is a young boy who voluntarily works for Vulcan.

Appearance
Yū is a young boy with long black hair that parts in the middle, leaving long bangs on both sides of his face, and one strand of hair sticking up in the middle of his head. He also has a braided ponytail with a white ribbon in it. He has light eyes, gray in the anime, and wears two plasters that cross each other on his forehead. He wears a black jacket with light buttons, cross insignia on the collar, black trousers and black shoes. He's also usually seen carrying a large rucksack. Under the jacket, Yū dons a blue sleeveless shirt with three white horizontal stripes on the torso, as well as white outlines across the collar and down the middle.

Personality
Yū is a very kind and responsible boy. Yū displayed courage when the Special Fire Force Company 5 teamed up on Shinra Kusakabe by defending him. He is very interested in mechanics and tinkering with machines, and being inspired by Vulcan's ability, voluntarily works for him. Yū is so determined to learn that he scavenges for materials for Vulcan since no companies will supply him with any.

VS. Special Fire Force Company 5 arc
While crossing the road, Yū is approached by an Infernal set out to kill him. Before being able to attack him, a Fire officer arrives and kicks the Infernal away. After Setsuo is apprehended by Company 5 and they pin Shinra to the ground, Yū pleads to them to not do anything to his rescuer before being pushed down by Tōru Kishiri.

Vulcan's Workshop arc
Yū meets Shinra, Arthur Boyle and Iris outside of Vulcan's workshop. Yū recognises Shinra when he saved him from Setsuo. Announcing to his master that he's come back with the parts he asked for, Vulcan yells at him for calling him master. Yū thanks Shinra for saving him and says that's what a hero does. Shinra asks if there is any cool stuff, to which Yū explains some of it was made by past generations. Yū explains he is Vulcan's disciple. Yū tells Arthur why he hates Haijima and the brigades. Shinra tells Yū that Special Fire Force Company 8 is different Yū explains that Vulcan is stubborn but he will try to ask him. Yū invites Shinra, Arthur and Iris in the workshop. Yū introduces Lisa Isaribi to the trio. Yū gets in trouble for bringing them to the workshop. Yū tells Shinra and Arthur that if Vulcan works at Haijima his dream won't come true. Vulcan shows his dream to Yū, Iris, Arthur and Shinra. Yū tells Vulcan that Haijima will destroy him and he won't be able to make any more inventions any more. Yū explains that Vulcan's father and grandfather were turned into Infernals at the same time and suspects Giovanni. Yū tells Shinra that Vulcan's grandfather had two disciples his father and Giovanni. Yu learns that Hajima robbed Vulcan his family of everything. Yū asks Vulcan if Arthur is going to alright. Vulcan orders Yū and Lisa to escape there a family until Liza betrays them. Yu wakes up and asks Lisa why is doing this. Giovanni strangles Yū in front of Vulcan until he gives up the key. Iris treats Yū for his injuries. Everyone in the workshop is treated for their injuries. Yū greets Iris and thanks the 8th for saving him. He gets questioned for what he did to the hospital bed and he will return to help Vulcan again.

Arthur's Adventure arc
Yū features prominently in this arc, traveling into the Nether with Arthur, Lisa and Vulcan, after the latter's promise to find Arthur some "orichalcum" to rebuild Excalibur was misinterpreted and quickly spun out of control.