Since the character has amnesia, it seemed like having a mimir to explain things and coach him on some of the Planescape jargon, etiquette and the world around him, as well as certain aspects on how to play the game, would be a handy thing to start him off with. He can diffuse the gloominess and edginess of the Planescape universe when things get a little too dark and depressing. Not content with merely giving the NPCs personalities, the team also decided to give them personal agendas. Chris provided an example in the form of the aforementioned Vhailor. We have an animated suit of armor in the game that is possessed by the spirit of its owner, a member of the Mercykiller faction named Vhailor.
Mercy is for the weak, injustices are rectified, and criminals punished - end of story.
And if a suit of armor can have a personality, it seems natural that other objects can too. It was so long ago, I can't remember everything about it, but one thing I did remember. In one of the chapters, Cobalt comes across one of his friends in the desert sitting by a campfire, his sword and shield propped up next to him. Musical notes are emerging from both the sword and shield, and this goes on for a page or two while Cobalt and his friend are conversing. The idea that equipment can be people too seemed to be a fun thing to play around with in Planescape.