Zack is the leader of the neighbor pack of Reiji Hidaka.
His ancestors and Reiji's ancestors had fierce fights over territory.
Chōmon no Tabi
Zack was born to the pack leader with his 3 siblings. Before they were even named, the curious puppies ran to explore and were found by a human.
The human took Zack and his siblings inside his knapsack and ran. Reiji saw this and chased the man with his minions. The scared human threw the knapsack into the river. Reiji rescued the puppies and thus peace was created between the two packs. Zack was named after the incident of the knapsack.
Zack respected Reiji a lot and eventually took over the pack as his father turned old and eventually died.
Reiji never returned which started rumors and restlessness.
Zack's neighbor pack seemed to fell on chaos and its members were starved and tired. Some dogs were even killed. Zack believed that Shin and his brothers caused this while in reality it was all because of Muramasa.
Gin shows up with Smith and Chūtora, when they found Tommy's corpse and give it back to Shin. Zack treats Gin and his friends as his guests though his pack doesn't have that much food either. Zack wishes the three would join his pack to fight Shin and explains the history of the area.
After finding out that the three are Ōu's dogs and when more Ōu's soldiers come, Sack joins them to help Shin's pack get rid of Muramasa. After Muramasa was defeated, Zack became great friends with Shin and peace was restored.
See more pictures in Zack/Gallery.