Chikuma Koshirou's prodigious talent was enough for his leader Ogen to include him among the ten elite warriors chosen to represent their clan of Iga Tsubagakure against the elite ten of the rival Kouga Manjidani clan, even though his standing within the Iga's hierarchy hed previously been that of a mere gennin (a ninja foot soldier).
Koshirou's technique enables him to create miniature tornadoes with his breath which suck his enemies in and then shreds them to pieces. He also wields a pair of kama hand scythes which Koshirou is able to use either in close combat or even throw like boomerangs.
After being blinded by Kouga Gennosuke his other senses adapt to compensate although he's nowhere near as capable with his blindness as Muroga Hyouma.
Koshirou and Oboro grew up together and despite the difference in their stations she sees him almost as an older brother. Koshirou himself is completely devoted to Oboro and may even hold romantic feelings for her. On the other hand, as Yakushiji Tenzen's apprentice he's obligated to follow his sensei's orders regardless of how he personally feels.
He's also the love interest of Oboro's bodyguard and confidant Akeginu although he's more or less oblivious to it otherwise. In the anime, Koushiro is an expert woodcarver and after being blinded, learns to carve with equal skill using his sense of touch alone.