One is a character who appears in Zero's Journey.
He is a brown dog with long ears, swirly hair, and a green scarf wrapped around his neck. When deceased, however, he shares a similar look to that of Zero, with a blanket-like body and a glowing aura.
Zero's Journey
One first appears in Mister Myzer's flashback, 10 years ago from that present time, where he playfully attacks his master and engages in a game of fetch with a worn elf toy. When his ball is thrown outside the window, however, One chases after it. He ends up in the street, distracted, before an oncoming Christmas vehicle runs him over, killing him on the spot.
One's death
While Mister Myzer expresses his anger and sadness for One's death, he is shown in a ghostly form watching his master, before sliding himself into the home as well, going unseen by him. It is likely One still exists as a ghost like Zero does.