After Mutikapa, the tiger goddess who lived in the forest surrounding the village is killed, Aruruu finds her cub and raises it as her own, naming it Mukkuru. He becomes Aruruu's best friend and never leaves her side, even being her riding mount in battle.

After Aruruu begins raising Mukuru, he begins growing very fast, reaching adult size in only a few weeks. Mukuru is always hungry and eats a lot, though Aruruu's motherly guidance keeps him from over-eating. Even after growing up, Mukuru always maintains the mentality of a cub, acting like a oversized kitten than a ferocious tiger unless in battle. Like his birth mother Mutikapa, Mukuru has a hide that is impenetrable to any weapon, even the Avu-Kamuu.

(Source: Wikipedia)