Shiro

is a dog from Jiraiya's novel, who appears within Tsunade's dream world.

Background
As a pup, Shiro was seen about to drown in a river until Naruto came to rescue.

Personality
From what has been seen, Shiro appears to be a very happy and jumpy dog.

Appearance
Shiro is a small dog with small black eyes and a small black nose. He has a light brown upper body and tail while his under-side is white. He has a small, circular white spot above each eye and his eyes are outlined with a thin white colour.

Jiraiya Shinobi Handbook: The Tale of Naruto the Hero
Shiro is seen with Itachi and Sasuke, when Naruto arrived at their house. Later, Shiro is seen with Sasuke, as he recalls a memory where Naruto saved Shiro before.

Trivia

 * When written as "白", "Shiro" means "white", while "城" means "castle" and "代" means "substitution".