He is black Kishu Inu.
With hearing great stories of their grandfather Akame and their ancestors. Muu like his brothers greatly admires and deeply respects Akame. He is also quite understanding when their mother forbid them from looking for Akame in hopes of keeping the next generation of the Iga clan alive.
Ginga Densetsu Weed: Orion
During the war with the Kougas, Akame decided to adopt all Iga puppies younger than 6 months of age as his own, which included Muu's mother. He made them swear to never return as an Iga ninja dog. After news about the volcanic eruption in Ouu, Akame is believed to be dead and he is allowed to go.
Muu travels with his brothers Rara and Shirozaru. First he encounters an Ouu soldier when traveling through the snow to seek out their grandfather. The Ouu soldier they encounter with is Sasuke, who is in trouble with Akakamakiri, Kamajirō and Kurokamakiri.
When Akakamakiri tried to attack Shirozaru, he grabbed Sasuke and along with Muu and Rara did a ninja jump kicking snow in the evil army's faces. His cousin asked Sasuke to lead them to the shed to help protect Weed's pups. But when they got there, the pups were nowhere in sight.
Muu and his cousins offer to help Sasuke round up the pups, and they set out to find them. The first pup they encounter with is Bellatrix, who is being kidnapped by an evil dog. Izō and his brothers are also there to help.
Ginga: The Last Wars
Muu accompanies his brothers to Ou when they hear about Monsoon attacking the Ou Army. The three dogs find Cross under the care of Daisuke and hear about the situation in full. When Cross notices Bob spying on them, she sends the brothers to chase after him and they succeed in catching up to him to question his motives. Bob escapes them once again and they chase him to the prison where the Ou dogs are being kept. The brothers help fend off the bears with Ogasawara Gennai and Zion.
Muu and his brothers follow Kawasemi as he leads the charge to finish off the rest of Monsoon's Kin. Sometime later after all of Monsoon's Kin is defeated, Akame collapses onto the ground and Muu and his brothers rush to their grandfather's side hoping he was alright, they watch their grandfather drift in and out of consciousness until he is revived by Daisuke and Hidetoshi realizing their grandfather needs help. He allows Daisuke to take his grandfather away for treatment with Izou.
Sometime later, Muu watches with his brothers and their comrades as the humans bury their fallen comrades along with paying their respects.
He and his brothers are reunited with Lord Gin and the rest of the soldiers; then the akita informs the group of some of their friends passing and is seen sleeping with everyone in the night until Gin wakes up from a nightmare and notices that their friends haven't returned.
- Muu’s name is a contronym, also known as an auto-antonym. As such, the meanings of his name contradict each other.