Pain

Pain is an Emotion from I Think, Therefore We Are.

Pain
A primal emotion, Pain is capable of speaking only a few words. Pain also is a key factor in the use of the hilt, as many of the weapons require Pain in order to form fully. The interesting thing to note is that Lady Shalice does not lose her Pain when summoning these weapons, so it is entirely possible that a previous owner's Pain is the source used to make the combination complete.

Theories speculate that is the pain of the God who created the item, while others believe that it belongs to one of those who stood in His name.