He's also called the White Demon and leads an army of Shepherds.
Mateus is a solid white Berger Blanc Suisse, who has red eyes with small, black curved markings around them.
Mateus received the order to invade the Kōga territory, seize it, and eliminate the remainder of the Kōga Clan.Mateus 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. When Orion and Izō encounter him in the Kōga lair thanks to Bon, Mateus introduces himself, saying that the Elder and the his followers are dead that the Kōga Territory was now his. This enraged Izō and he attacks Mateus. However, their fight was quickly interrupted when one of Mateus's followers entered the cave. While he was distracted Orion and Izō tried to escape by the entrance. Mateus, still having the urge to fight, peruses 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. Mateus then sets off with his army to find the Ōu soldiers.
When they did, Blanca and his small army perused them, while Mateus 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, Mateus swore 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 Sherperds, tired of searching the whole area, returned with no success. Mateus ordered them to search again, but received an unexpected visit: Masamune, accompanied by Kenshin Nokizaru and a small detachment, arrived to meet Matheus. Matheus welcome greets them and suddenly Masamune collapses from his neck wound. When Kenshin explained to Mateus what had happened, Mateus, accompanied by Kenshin and a small dision of his Shepherd army decide to carry Masamune to the secret Kōga hot springs to heal.
While Masamune is healing, Mateus 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 Mateus for what had happened, but forgave him when Mateus pleaded that he had nothing to do with the assassination attempt. For a while, Mateus and his Shepherd army joins Masamune's forces during Masamune's voyage toward Shikoku. Matheus becomes Masamune's adviser and replaces Kenshin for the time being. Whilst Masamune is fending off the Ou army, Mateus 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. Mateus 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 Ōu 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 Battōga. Mateus cries as he watches his leader kill himself and tries to commit Seppuku himself, only to be stopped by Unsai. Bon says that Mateus is no longer his father as he has committed several crimes and has no sense of justice anymore. Mateus continues to cry as the Kōga ninja dogs arrive and tell Mateus he should give their underwater base back as he stole it from them. Mateus attempts to run off and try to kill himself again, but his actions are thwarted once more by the Ōu army, who show him mercy. Realizing the mercy of the Ōu army and seeing that his son is in a better place he admits that he doesn't have the right to raise a child, Mateus looks back at Bon one final time and tells him to take care of himself. Mateus wanders off with his army until they are out of the Ōu Army's sight, they keep running until they make it to the forest and Mateus collapses. His subordinates ask him what they should do now but all the white Shepard wants to do is die but they refuse to let him do that. Realizing that Futago Pass is the paradise of the Ōu Army, Mateus knew he couldn't set his paw on it, but found another way to help by supporting his son and the Ōu Army from afar.