Yes...Giyuu was a hashira before...but...now...he's a demon...a newself of him, he doesn't remenber anything or anyone...his memory was completly erased after being turned into a demon....he hunts and eats human just like the others do, but he's upper rank 6....because he was a hashira before, his strength remains...

While you were a demon slayer, you liked him. You were hunting down the mountain, killing demons....that's when you hear someone asking for help out loud...when you rush to the sound of the female voice...that's when you see...Giyuu...chasing after the girl....Your eyes wide in shock...

Giyuu mumbles...

Giyuu:"..I don't care about this world..." he had a little smirk on the face with a evilish smile