Masamune is a rather unusual looking dog. He has a round head with pale human-like hair on the top of it. His fur is an orange-brown colour, and the fur is paler on his chest, belly, legs, as well on his face. His muzzle has a darker color.
Masamune's right eye is missing, while his remaining eye has a small pupil. This eye is also decorated with eyelashes, whilst his missing eye is surrounded by spots.
He wears a black scarf around his neck and head, and on his lower forelegs he wears black gaiters with white ribbons. He uses his scarf both offensive and defensively, some uses Masamune displays in the manga are among others; blinding the enemy, tripping them over, strangling them and to lower the damage of an attack.
Masamune is ruthless, smart and clever, and has very good intuition. He doesn't have any problem killing other individuals, even if they're family.
As a puppy, Masamune was a hard worker and was well respected among his comrades. Masamune worked hard, wishing to make his father Terumune proud. However, when Yamabiko was born he was at first happy to have a sibling, but it later turned into anger and hatred when he found out that Yamabiko was going to get everything Masamune worked hard for, even his father's love.
His anger grew so strong that he betrayed and killed his own family just to maintain leadership of the clan.
After Masamune gained leadership of the clan through combat he knew it'd be temporary until Yamabiko became of age. Despite being evil, he still believes in honor as a ninja dog. Rather than receiving a death blow he would take his own life.
Masamune is very skilled and strong, and can with ease cut the head of someone. One of the techniques he wields are the Ningatō, which he uses to decapitate enemies.
Ginga Densetsu Weed: Orion
When he was still just a pup, Masamune's owner, Kurohabaki Jinemon, gouged his eye out and left him in the mountains to die.
Luckily, Terumune, the Kurohabaki Clan Leader, finds him and adopts as his own son. In addition, his followers respected him greatly and his father trained him relentlessly to become a strong fighter. For Masamune, this was a difficult task, but he didn't give up and tried his best to prove to his father that he was worth of a fighter.
However, his long efforts were in vain when Yamabiko, who was to become his "younger" brother, was born. From his birth, Terumune looked after Yamabiko more than Masamune and had already decided that his son shall succeed him to obtain the leadership of the clan.
This made Masamune angry and thirst for revenge. He then banished his father and took the leadership of the clan. But his revenge wasn't accomplished, for he had to kill his younger brother, who was sent to Shikoku to avoid battle.
After the volcanic eruption in Ohu, he takes over the utopia and convinces other dogs from nearby areas to join and strengthen his huge army of Kai-Kens. On his journey to find more dogs, he sends out Masashige to kill anyone in the local area who wasn't willing to join, resulting in Kisaragi's death. Masamune also gets his revenge by pushing Kurohabaki Jinemon off a mountain cliffside.
As Masamune continues his travels, he is trapped and ambushed by Tesshin, who used a wounded Masashige as bait to get close enough to attack. Tesshin uses his Battouga on Masamune, which Masamune quickly wrap his scarf around his head, decreasing the damage of Tesshin's attack. The Battouga seems to only made a small wound, but later affects Masamune a lot more than right after the hit. He flees from the battle and is later greeted by Matheus. However, the Battouga takes its toll and Masamune collapses. Matheus takes him to the hotspring to heal Masamune's wounds. While resting there, he is attacked by Trigger, a member of Matheus' group.
Masamune kills Trigger and scolds Matheus for his attempted murder. Despite this, Masamune continues to let him travel with him, and they leave the hotspring. Eventually, they run into Gennai and a fight quickly breaks out. Masamune gets the upperhand over the large dog and hangs him from a tree by his scarf, only letting go when Gennai stops breathing. Masamune leaves the dog for dead and heads off and continues to travel with his group.
A short time later, he encounters the Ou army by the edge of a cliff. Masamune sends Matheus away to fight the oncoming army, led by Weed, while he stays to fight the others and kill Yamabiko. Masamune is attacked constantly by the Ou army, but he evades all attacks, and manages to kill a few dogs. Masamune eventually got attacked by Gin who drags him into the river below. Masamune nearly drowns Gin, and escapes back up the cliff to freedom.
He tries to kill Yamabiko in a rage, but is stopped by Rigel. He attempts to throw Rigel over the cliff but Rigel is saved by Unsai. Masamune is attacked by Orion, but manages to escape and find Matheus again. Kenshin, who has disowned him, attacks but is wounded severely. With Kenshin injured, Masamune is once again surrounded by the Ou army. Masamune tries to attack Weed but Yamabiko gets in the way, only to be nearly killed in the process.
Masamune, determined to finish his work, charges to kill Yamabiko but is stopped by Orion and the others. Weed attacks him and flings onto his back, then orders his group to surround him. Although Masamune begins to attack again, Weed decides he will not kill him and lets him go as long as he lives a peaceful life. Orion, in rage, uses his Battouga on Masamune, hitting his head but not wounding him severely. Weed is shocked by Orion's actions and disobedience of the army's laws.
Orion fights back saying that Masamune ordered the murder of Izou's family and others, and should be killed. Weed tells Orion to leave the pack, as this fight is not for him. Orion cries in sadness and rage and begins to run away, only to be stopped by Andy.
Masamune stands up and tries to fight, but the Battouga takes effect and causes him to collapse and cough up blood. Unwilling to die by the Ou army's hands, he bites his stomach and tears his intestines out. Matheus witnesses the event and cries as Masamune dies from committing Seppuku.
- Yoshihiro Takahashi has mentioned in an interview that the Masamune is stronger than Hōgen.
- In an interview, it was revealed that Kurohabaki Masamune (and the basic plot of the series) is based on the historical Sengoku era warlord, Date Masamune, who was also known as dokuganryū, or "one-eyed dragon," for his missing right eye. Kurohabaki Masamune, like Date Masamune, has a missing right eye. Similarly, Masamune had a ninja group called Kurohabaki under his command and an ambition to conquer Japan.
- Masamune committed Seppuku, a Japanese suicide ritual used by Samurais in which the users would cut their stomach open and allow themselves to die by their own hands rather than being defeated at the hands of their enemy and upholding their honor.