[X] Put it through the back of the Hollow's head, killing it. You're ending this cleanly and simply, you don't want any loose ends or to traumatize the Iwabas or Mai, even if you're going to have a serious talk with both of them later.
[X] Lodge your knife under the Hollow's mask and rip it off. You're going to see its face, to see the truth behind Souls and Hollows. The group you're with might not appreciate it, but you've been used, and you have a desire to see the truth with your own eyes.