Arnold

Arnold is the ghost of Reimi Sugimoto's pet dog featured in Part IV: Diamond is Unbreakable. Arnold remains by her side as she bides her time in the alley and later defends her from Kira after Kira dies.

Appearance/Personality
As a guard dog, Arnold is big and strong, but friendly to strangers (as he didn't attack Rohan and Koichi). Loyal to his master, he remained by her side even in the afterlife. He notably has a huge gash across his throat dripping with blood as a result of his death.

History
Arnold was Reimi´s guard dog. One night, Yoshikage Kira invaded their house and slashed his throat. Picking the dog´s dead body, Kira hanged him on the clothes hook in the wall, his neck almost cut off. His blood remained dripping for more than 10 minutes before anyone noticed his death. Living as a ghost bounded with an alley, Reimi and Arnold wait the day they can find someone who could hear their story and report Kira to the police before more teenage girls are killed.

Diamond is Unbreakable
Reimi and Arnold finally finds someone with a supernatural ability who can see them: Koichi Hirose and Rohan Kishibe. After telling their story Rohan agrees to help after knowing the killer is still alive, and so he warms the stand users of Morioh about him. Arnold then remains by Reimi´s side as she bides her time in the alley.

After Kira's death his soul is transported to the ghost alley, Arnold defends Reimi from Kira, biting him. This makes him look back and be dragged to the afterlife by hundreds of hands. Finally in peace, the ghost alley Arnold and his owner used to live in disappears and both presumably ascend to Heaven.