Rukia: I'll say one thing: Crack its skull from behind with a single attack. That's the fundamental of Hollow vanquishing. Do not forget.
Ichigo: What's that mean?
Rukia: To reduce damage from fighting... and to keep from ever seeing the Hollow's identity.
Ichigo: What do you mean "Hollow's identity"?
Rukia: You saw it too, didn't you? Hollows.. were all formerly human souls.
Ichigo: (surprised) You never told me that! He's not a monster? He's a monster we have to take down, right?
Rukia: That's right. Now he's just a monster. You have to take him down.
Ichigo: But he used to be human...
Rukia: Be it hatred or sadness, the souls of those who still have feelings for this world, tend to not accept burial by us and are left to wander. Then, perhaps on their own initiative, are surrounded by those who have become Hollows, and become new Hollows.
Ichigo: It can't be...