He is a purebred German Sheperd with a black and light cream fur color.
Maru is much like his father. Upon his mother's death, he was willing to sacrifice himself to fight the bears as his father was, which led to Jerome stopping the attack. Like his brother, Sunny, Maru is rational yet hasty as he attacked Bob with the intention to kill but stopped when told to.
He blames himself that he couldn't protect his mother or siblings despite being a male.
In the end of GDW, after the hybrid bear crisis, Jerome and Lydia got 6 pups: four males and two females. One of them was Maru. He and his siblings were born in autumn in Futago Pass, three days after the birth of Weed's and Koyuki's pups Bellatrix, Orion, Rigel and Sirius.
When a volcano erupts in Ōu, the dogs are forced to flee. Maru is too small to run fast enough and is carried by the Ōu army to safety but they pass out from the smoke and ash in the air. Jinemon Kurohabaki finds them and takes them to an abandoned mine where he locks them up. Maru and the others are later released by Daisuke who acquires the key from Jinemon.
A few months have passed in Ōu when the son of Akakabuto, Monsoon, attacks they Ōu Army intending to kill them all and take back his fathers land. Maru is with his mother and siblings when they are suddenly attacked by red backed bears and are forced to flee. His brother, Lenov, is crushed and killed by one of the bears along with Akira and Tetsuo who try to help their brother. Lydia is badly injured as well and carried to the top of a hill where Gin is calling for everyone to gather. Maru and his surviving siblings cry over the body of their mother as she dies from her injuries. When his father, Jerome tries to make a suicide attack on Monsoon, Maru joins in but is stopped by Kurotora and Rocket. Ultimately, Gin orders the females and pups to be taken away from the battlegrounds to safety. Maru refuses to leave his mother behind and stays with Sunny to protect her body. Tonov is taken to safety by Reika whilst Sunny and Maru are led to safety by Cross, but the three are constantly hounded by Bob, a dog working under Monsoon. After dealing with Bob, Cross takes the two pups to Daisuke's house where they can hide. Daisuke is kind to the dogs and gives them food and water while they hide under his porch. However, Bob tracks them down and tries multiple times to attack and kill them. Despite Cross's age and Maru being young, the three dogs easily defeat Bob each time, forcing him to retreat back up the mountain. Maru remains with Cross and Sunny under the protection of Daisuke.
Sometime later, the rainy season has started and Maru mentions he can smell his father's scent. Jerome appears to their relief and tells Maru that, as a male, he should be fighting to protect his family, but is nonetheless happy to see his son and daughter safe. Sunny and Maru are happy to see their father and are eager to help. Cross stop Maru, Sunny and Jerome, Jerome cannot travel far because of his wounds so Sunny and Maru go to Ōu and find the injured dogs. They found injured Ōu army and Kurotora.
They tell him that Jerome made it safely to them and they're there to help, and if they were to die they would be reunited with their mother and three siblings. Kurotora flies into a rage, calling them fools for wanting to throw their lives away so suddenly.
Sunny told Maru not to let him go, for he's going to rip his throat out. Before they can, Tesshin intervenes, saying they need to listen to him before killing him. Sunny reluctantly lets Bob talk. Bob suggests that the bears are not afraid of dogs. Even if the dogs were to attack them in high numbers, Monsoon would welcome them, because the dogs are just food to them. Maru and Sunny sees this as a threat, but lets Bob go and tells him to speak more clearly and that there's no use killing someone like him.
After he sees the bears flee Maru and along with Sunny leaves to follow Akame agreeing that the war isn't over yet.
Sunny and Maru manage to catch up with Orion's group and Kawasemi leads the rest of the army in the attack against the bears. With the help of the hunters the rest of Monsoon's Kin is defeated for the moment the brothers celebrated in a victory howl with their comrades believing that they have avenged the death of their siblings and mother Lydia but the victory is cut short by Orion because he knows Monsoon is still out there.
Maru is also alerted when Akame can't continue the fight, he hears from Kyoshiro that as long as he's around there will be no more fighting amongst each other. Some time later, he and Sunny joined in on the celebration feast with the others.
Then, Maru is with Sunny and Cross as they go to pick up Ken, when the great dane gets embarrassed about being licked by his mother. The brothers remind him that they used to get licked too and when Ken tries to apologize about being unable to protect their mother or siblings Maru agrees with Sunny saying that it was their fault and thanks their mother for what others have done.
As the two brothers take Ken back to the other soldiers, Maru offers to carry Ken on his back while Cross informs them that everyone is doing fine but she doesn't know extent of their injuries until Gin corrects saying that Weed, Akame and everyone is doing well but also tells them the passing of Hiro, The Koga Elder, Kenshin and many more. Saddened with the loss of their friends Sunny follows the group until they find a place to sleep.
He and Sunny see their father Jerome return to Futago Pass with Daisuke following him. When Daisuke comes to take Gin back and sees that he brought a leash. The pack runs away, and Maru bites onto the leash thinking that Daisuke was hear for them all, until Jerome tells him to let go. His father tells him that Daisuke is their friend. Watches Lord Gin return with his former owner.
Shortly, after Gin leaves he and the others see Orion run and roll off the cliff and into the water. Maru and Sunny watch in shock as their father Jerome purposely falls after Orion but is relieved to see that he and Orion are okay.
After his father and Orion return to Ohu Maru continues searches for the sickle with his friends but see it's no where to be found.
Seeing Orion's skills, Maru and the others want to try it too, but when they try to take a swing, they don't manage to do the same as Orion. He hears from Orion what he learned in a day took Akame two years to learn. As the training continues, he becomes impressed with Orion's progressed and gets encouraged to keep training. Then, he hears that Rigel and Rocket have returned, but with them are a bear cub. Everyone charges at it, after Kyoshirou attacks the bear cub, he becomes surprised that Rigel flipped Kyoshirou over with ease. After Rigel explains everything, they all become surprised that Sirius wants them to raise Chibi until he becomes an adult.
Maru watches Rigel and Rocket convince Orion not to kill Chibi. Then, he hears Kurotora batter Orion for his attitude. When he hears that Orion is trying to protect everyone in Ohu Kuro says that he doesn't need protecting from him. He also sees Orion pointing his bamboo stick at the kai ken convincing the pup to attack him but is stopped by Jerome and watches in horror as Orion almost attacks his father. He watches Kyoshiro give Orion a beat crying in tears and Mel restrains him but soon the tears turn into laughter as everyone runs back up the hill.
After Monsoon's defeat Maru returns to Ōu with his brother Sunny and father Jerome, soon nightfall comes and the two come across a rundown car thinking it's a great place to sleep but soon see it's already taken by an elder dog but was surprised to see him run off with his granddaughter.