Matamune is a bakaneko and a spirit who fights along side the Asakura family against Hao. He is manga exclusive to Shaman King.
Before Matamune became a great spirit who lived for a thousand years, he meets Hao Asakura as a stray. Hao gives Matamune a necklace with three bear claws that has Hao's mana. This allows Matamune to live as long as Matamune does not exhaust Hao's mana. In the flashback (vol. 19-20), it is explained by Kino, that Matamune had regrets after killing Patch Hao who is the reincarnation of his master with the help of Yohken Asakura.
When Matamune meets Yoh for the first time, Yoh states that Matamune likes catnip, likes to read books, and write poems. Matamune is Yoh's first spirit ally, and he dies when Yoh breaks the seal on him. The broken seal allows Matamune to become ghost for Yoh to integrate with. After all the mana is spent on killing the demon, Matamune passes on into the afterlife. Before Matamune pass over to the other side, Yoh promise him that he will defeat Hao and become strong enough to summon him.
Matamune, who is created by Hiroyuki Takei, has no current information on what inspires the author to create him.
"Fear is often worse than danger itself. Worry feeds upon itself. The only cure is action."
Matamune - Vol. 19 - First quote when meeting Yoh and Tamao for the first time.
"I often wish that I could just get over the things I've done. Our reasons may differ...but the more we try to forget...and the more we struggle, the more we become entangled."
Matamune's regrets - Vol. 19 - to Yoh Asakura
Matamune does not change much, but only his view of the world and his life change when he meets Yoh Asakura. He finally rests in peace after letting go of his regrets and helping Yoh to slay the demon.
He appears in Yoh's flashback (volume 19) after Yoh has agree to Iron Maiden Jeanne's request to drop out of the tournament.
After Yohmei has told Yoh that they have found his future bride; Ponchi and Conchi is teasing Yoh for getting a bride at an early age, Matamune's voice appears to tell Yoh to meet his bride to get over his fear. As soon as Yoh and Tamao sees Matamune for the first time, he asks Yoh to quiet down the vulgar spirits. He tells Ponchi and Conchi to wear some pants. When Ponchi and Conchi thinks Matamune needs to show some respect, Matamune asks them if they want to fight. As Ponchi and Conchi charge at Matamune, he easily defeats them.
Matamune tells Yoh that he has served the Asakura family for 1,000 years, he is his humble servant, and he likes catnip. During Yoh's narration, Matamune hhas convinced Ponchi and Conchi to wear diapers. At the train with Yoh to Aomori, Matamune asks Yoh if he has friends due to Matamune's observations of Yoh's personal belongings. Matamune knows Yoh does not have friends. As Yoh tells Matamune about him lying to his "normal" friends at school to act normal, he gets surprised by Yoh's answer. Yoh says he will change the way people look at shamans if he become shaman king. In Aomori, Matamune is asked by Yoh about visiting Aomori and Mt. Osore. Matamune replies that this is his ninth visit to Aomori. For Mt. Osore, he says that he will not go there because Mt. Osore is a bridge to the afterlife, a place for the living to mourn the dead, and a place for ghosts who do not have a home. Even though Matamune shares the ghost's pity, he gets a second thought to visit Mt. Osore since he is a ghost who has no home.
At a restaurant, Matamune tells Yoh about his grandmother, Kino. He met Kino when she was first wed into the Asakura family about 50 years ago. He tells Yoh that Kino's eyesight was robbed by war. When Yoh asks Matamune about war, he dodges the question and tells Yoh to look in history books. Also, he tells Yoh that it is best to forget these things. Matamune decides to leave Yoh to visit Kino.
At Kino's inn, Matamune spends some time with Kino. He remarks how Kino's tongue is always sharp and how the place has never changed. When Kino tells Matamune that he has the greatest strength of all the Asakura's spirits, Matamune asks Kino about Anna's condition. He learns that Anna is serious force due to her being cursed by her birth, her powers, and her fate. Back to Yoh on the bridge, Matamune finds Yoh running from a demon. After Matamune explains to Yoh that demons are manifestations of human anger, resentment, and regret, he quickly kills the demon. Before Matamune and Yoh leave for Kino's place, he senses Anna's mana and knows she is there. As Matamune and Yoh arrive at Kino's place, Matamune watches Yoh struggle with the thought of Anna after she appears briefly.
In the baths, Matamune tells Yoh to calm down after Yoh thinks Anna will not love him back. When Yoh asks Matamune if that demon was involved with her, Matamune thinks so but does not tell Yoh. Matamune tells Yoh not to dwell on bad things; he tells Yoh that he, too, wish to get over his regrets.