He's also called the White Demon and leads an army of German Shepherds.
Matheus is a solid white German shepherd, who has red eyes with small, black curved markings around them.
Ginga Densetsu Weed: Orion
Matheus received the order to invade the Kōga territory, seize it, and eliminate the remainder of the Kōga Clan.
Matheus and his minions chase the Kōga Elder and his 3 remaining subordinates and trap them near the edge of a waterfall. Matheus demands them to surrender but the Kōga dogs jump off the cliff into the waterfall.
Thinking that the shinobi dogs died in the fall, Matheus's mission of ceasing the Kōga territory was success and decides to settle his pack in the territory for further orders from Masamune. Also, the secret underwater Kōga cave becomes his lair for the time being.
This enraged Izō and he attacks Matheus. However, their fight was quickly interrupted when one of Matheus's followers entered the cave.
While he was distracted Orion and Izō tried to escape by the entrance. Matheus, still having the urge to fight, persues them with his follower right behind him. An expert swimmer, Matheus managed to quickly pass Orion and Izō and waited for them at the exit of the lagoon. He then attacks Izō in an attempt to drown him while his follower did the same to Orion.
Suddenly, the Kōga Elder and his followers quickly came the rescue and Matheus and his follower were forced to retreat. Later, he encounters Blanca and his small division, whom were thinking that Matheus was in danger. Matheus then sets off with his army to find the Ōu soldiers.
When they did, Blanca and his small army persued them, while Matheus stayed with Bon. At nightfall, Matheus was shocked: his followers returned with Blanca's body, who was fatally hit by Orion's Battōga. When his soldiers explained what had happened and that the Elder and his minions were still alive, Matheus sweared that he'd avenge Blanca for what they did to him.
Later that night, he scolded one of his soldiers for sleeping while keeping watch of the entrance of the cave. They were soon interrupted when 2 of Kojūrō's followers arrived and asked him if he could support Kojūrō and Yamabiko's Clan against Kamakiri Brothers's pack's invasion of Shikoku.
As a response, Matheus said that he's serving under Masamune and ordered his men kill them, but were unsuccessful when Tesshin and Toshimitsu showed up and saved the two dogs. Their search lasted until morning and the German Sherperds, tired of searching the whole area, returned with no success.
Matheus ordered them to search again, but recieved an unexpected visit: Masamune, accompanied by Kenshin and a small detachment, arrived to meet Matheus. Matheus welcomly greets them and suddenly Masamune collapses from his neck wound. When Kenshin explained to Matheus what had happened, Matheus, accompained by Kenshin and a small dision of his Sherperd army decide to carry Masamune to the secret Kōga hotsprings to heal.
While Masamune is healing, Matheus hears Bon's scream from a distance. Fearing that his son is in danger, he gives his follower Trigger the command to watch over Masamune and sets out to find Bon. For a while he calls for Bon, but with no answer. Suddenly, one of his follower arrive saying that Trigger attempted to assassinate Masamune. When he returned, Masamune scolded Matheus for what had happened, but forgave him when Matheus pleaded that he had nothing to do with the assassination attempt.
For a while, Matheus and his Shepherd army joins Masamune's forces during Masamune's voyage toward Shikoku. Matheus becomes Masamune's advisor and replaces Kenshin for the time being. Whilst Masamune is fending off the Ou army, Matheus finds a group consisting of George, Ken and the others. He begins fighting them but stops when Kenshin and Weed arrive to help fend them off.
Matheus orders his group to kill everyone but is tackled by Bon, who is trying to stop him. Matheus can't believe his own son would attack him but nonetheless attacks back, only for Akame to step in and save the young pup. When Matheus runs off with his group to avoid the fight, he comes across Masamune trapped by the Ou army.
Masamune screams his anger at the dogs and decides to commit Seppuku rather than die from the injury to his head given by Orion's Battouga. Matheus cries as he watches his leader kill himself and tries to commit Seppuku himself, only to be stopped by Unsai. Bon says that Matheus is no longer his father as he has committed several crimes and has no sense of justice anymore.
Matheus continues to cry as the Koga ninja dogs arrive and tell Matheus he should give their underwater base back as he stole it from them. Matheus attempts to run off and try to kill himself again, but his actions are thwarted once more by the Ou army, who show him mercy. Realizing the mercy of the Ou army and seeing that his son is in a better place he admits that he doesn't have the right to raise a child, Matheus looks back at Bon one final time and tells him to take care of himself.
Matheus wanders off with his army until they are out of the Ohu Army's sight, they keep running until they make it to the forest and Matheus collapses. His subordinates ask him what they should do now but all the white german shepard wants to do is die but they refuse to let him do that. Realizing that Futago Pass is the paradise of the Ou Army, Matheus knew he couldn't set his paw on it, but found another way to help by supporting his son and the Ou Army from afar.