Training since his youth, Koshiro is Tenzen's incredibly clever and skilled pupil, who is able to create massive vacuums just by sucking in his breath. It is clear that he looks up to Tenzen because, although he is like a brother to Oboro, he is willing to betray her trust and try to kill Gennosuke on Tenzen's command.

(Source: FUNimation's Official Basilisk Website)