Well, well, well... If it isn't mother Amaterasu. It's been a long time. But with all these monsters around, the only place I could hide was in this shrine dedicated to ancient heroes. If there is anything I can do to assist you in your endeavors, I'm at your service. Make good use of this sword, as it was designed to conquer evil.
He is the Celestial Brush god of Power Slash. He takes the form of a rat with the same symbolic markings as Amaterasu, while armed with a huge sword, being an ancient Jomon-style tsurugi, with an underestimating red sheath of small size.
His constellation can be found in the Cave of Nagi, which lies in the River of the Heavens. When idle, he sticks his sword in the ground and balances on the pommel. If Power Slash is used, the kanji 「斬」 (「き」?; ki; chop) appears. Oddly, Tachigami's constellation looks very similar to the in-game representation of ki.
Power Slash is perhaps the most useful and versatile Celestial Brush technique. Draw a horizontal line to damage enemies, knock down people, cut down vegetation and rocks, ring bells, shatter breakable objects, and deflect projectiles. Its power can be upgraded to inflict more damage and cut through harder objects. However, some enemies (particularly boss characters) are immune to Power Slash until their weak point is exposed or, in some cases, are completely immune to Power Slash.
Power Slash is the only Celestial Brush technique which does not create Holy smoke besides Cherry Bomb, Veil of Mist, and Galestorm, as these are the only techniques which can be used whenever the player wants and which do not require a source.
↑"Rending God: Oh, our Merciful Mother, Ōkami Amaterasu... With the mortal realm overrun by the numerous demons, I was forced to cower within this tiny shrine constructed in honor of a great warrior from long ago. If it is to be my duty to aide you, the greatest of the Gods, please grant me my sword of exorcism, which I will use to undo this evil!"—Tachigami (Image Scroll of Those From the Road, page 18, Ōkami Official Complete Works)