Perverted teenager and newest student in the now co-ed Tenbi Gakuen.
Prior to his entrance to Tenbi Academy, Takeru was the childhood friend of Haruko Amaya and Inaho Kushiya. HIs mother was the master of a dojo, and former member of Maken-Ki herself, but upon her death Takeru was taken away, and spend the next three years in an all boys school. When the time came for Takeru to choose a high school he settled on Tenbi Academy simply for the facts that it required no interview or testing, and that it was a co-ed school.
Takeru Ohyama was created by the creator of Maken-Ki: Hiromitsu Takeda.
Takeru begins his first year at Tenbi Academy without any real knowledge about just what type of school Tenbi is. His only knowledge of the Academy is that his childhood friend Haruko Amaya is a student there, and that it is a Co-Ed school. He learns quickly that Tenbi academy is much more than it seems when he gets lost at the school on his first day and wanders into the middle of a duel between Academy students: Azuki Shinatsu and Garreto Kinua. The duel between them is not a simple martial arts match, but rather a battle fueled by magical artifacts known as Maken, and a magical force controlled by the students known as: Element. The battle between the two students is enough to shake and break up the ground, and this catches Takeru a little off guard at first. That is until he meets Kodama Himegami who is hanging upside down from a tree, and he informs her through sign language that he can see her panties. Himegami falls from the tree on top of Takeru, accidentally kissing him, while straightening herself she notices a mysterious mark of his chest and declares to herself that Takeru is her "mortal enemy".
At Tenbi's Orientation, Takeru is placed into a duel against Himegami and though the fight starts off normal Himegami attempts to land a mortal blow, but not before another student by the name of Inaho Kushiya intervenes and rescues Takeru. Inaho claims to know Takeru and states that she is his fiancee though he has no memory of it. When Takeru gets his dorm room at Tenbi he is roomed with Inaho, and Himegami decides to room with Takeru as well to keep an eye on him, and learn his secret and the truth about the mark she saw on his chest. Haruko expresses great disapproval at this and moves in with Takeru and the other girls as well.
Takeru becomes a member of Maken-Ki, which is the schools security force who's duty is to monitor the duels between students, and to be certain that no unauthorized duels are taking place. Takeru becomes an impossible case right from the start for not being compatible with a single Maken, magical artifacts used in duels by the students, and is forced to wait a month before he can have a Maken of his own by the Maken inventor: Gen Tagayashi. Takeru is then forced to be accompanied at all times by a Maken user so he will not become an easy target for the other students.
Over the following months Takeru becomes labeled as a pervert by the girls for particpating in various lewd events such as peeping on the girls stripping down for their physicals to being one of the students behind a petition for co-ed swimming classes. He is often accompanied in these endeavors by fellow student, friend, and Maken-Ki member Kengo Usui. Takeru also comes into contact with a mysterious enemy known as the Kamigari who send out mercenaries to attack Tenbi's students, and a mercenary organization known as Venus who intend to find and stop the person attempting to revive Yamato-No-Orochi. Takeru fights a Kamigari mercenary and reveals that he has a special ability known as Blood Pointer which allows him to absorb another's element through his mouth thus making him stronger. He also eventually recieves his Maken, called Over blow, which causes a another's maken to overflow with element and loose control of it.
At the end of the first series he attempts to rescue Haruko from Kimigari, but instead awakens as a Yamato No Orochi, and attacks Tenbi. Haruko is rescued by Venus and uses her Maken: Marakumo to bring Takeru under control. Takeru is approached after he regains control by the Kimigari member who caused his transformation and invited to join Kimigari. Takeru turns the offer down, and promises to put a stop to Kimigari's actions, and begins training to prepare himself for the future.
Initially Takeru is incompatible with a Maken, but after a month Gen Tagayashi develops the Over Blow maken. This maken allows Takeru to cause his opponents to swell with power, to the point to where they can no longer stay in control their own maken.
Takeru is never truly aware of this ability, and it first awakens when he protect Himegami from a Kimigari mercenary. The ability allows him to absorb element through is mouth, and become stronger as the result.
Maken-ki!: It's summer! Swimsuits! At a camp! (OVA)
Maken-ki!: It's summer! Swimsuits! At a camp! is an OVA that was bundled with the limited edition of the eighth volume of the manga.