Bull

is a ninken and a personal summon of Kakashi Hatake.

Background
In the anime, after they made a contract with Kakashi, Bull and the other ninken aided him to search the abducted Rin Nohara by the Kirigakure shinobi and acted as a diversion in order to allow them to flee. They also helped searching for the fleeing Orochimaru.

Personality
Befitting of his name, he is a large, intimidating black bulldog wearing a spiked collar. He and Pakkun seem to get along well enough for him to allow the latter to ride atop his head. Comparatively, Bull is the largest of Kakashi's ninken.

Appearance
Bull is the largest summon in Kakashi's ninken pack. He has dark fur and wears a spiked collar. He also wears the standard outfit of Kakashi's ninken which consists of a blue vest with the heno-heno-moheji seal and a Konoha forehead protector worn on his left front leg.

Abilities
Bull possesses heightened senses with which he can use to track his target with ease. He has also demonstrated the ability to track his target underground through the use of the Summoning: Earth Release: Tracking Fang Technique as well as hold a target down, allowing Kakashi to attack them freely. He also has has exceptional teamwork with the rest of his pack, able to develop and effectively use several offensive, and defensive formations in battle which make use of the manoeuvrability and powerful limbs.

Prologue — Land of Waves
Kakashi called on Bull and the ninken pack to track and capture the missing-nin Zabuza Momochi. They managed to bite onto the missing-nin and pin him in place. This allowed Kakashi to use Lightning Cutter without fear of a counter attack.

Sasuke Recovery Mission
A short time later, Bull and his fellow ninken would be called upon by Kakashi to track down Naruto Uzumaki and his team who had gone off in pursuit of Sasuke Uchiha.

Sunagakure Support Mission
In the anime, he along with the other ninken were summoned by Kakashi to investigate the Takumi Village.

Kazekage Rescue Mission
At the start of Part II, Bull and the ninken pack were called upon to track down the Akatsuki member Sasori using a small piece of his clothing.

Itachi Pursuit Mission
Several weeks later, Kakashi summoned the pack to help track down Itachi Uchiha. Bull was partnered with Naruto, Hinata, and Yamato. During their search, they encounter Kabuto Yakushi, who had just integrated Orochimaru's cells into his own body.

Kakashi Hiden: Lightning in the Icy Sky
Kakashi summons Bull and the other ninken to search for the exploding tags planted around the Tobishachimaru. Once they gather all the tags, the ninken give them to Might Guy.