Demon Slayer: The Kamen Rider Decade Breathing Style

[Genius protagonist, dual heroines, Kocho Sisters, doing everything possible to avert tragedy] [Though it includes Kamen Rider elements, the protagonist still uses Breathing Styles, respecting the worldbuilding] Tsukasa Godai unexpectedly encounters a bat named Kivala. According to her, the birthmark on his waist actually contains the power of a Kamen Rider named Decade. With Decade's power, in addition to creating his own Breathing Style, Tsukasa Godai can obtain cards for other Breathing Styles by earning the swordsmen's recognition. To use this power, after passing the Demon Slayer Corps selection, Tsukasa Godai commissions a swordsmith to forge a Nichirin Blade fitted with a card holder. Inserting a card into the holder on the Nichirin Blade lets him use the Breathing Style corresponding to that card. Besides this, to repay Tsukasa Godai for saving her life, Kivala grants him two abilities. One lets him briefly enhance his physical abilities when he takes on his Kiva form; the other helps him awaken supernatural powers that can evolve into Agito. Unfortunately, this world's rules do not allow Kamen Riders to exist, so transforming is impossible. Still, these abilities are more than enough.