Rune Factory: Befriending Bosses

I know this game's been out for a while, but I only just picked it up. It looks to be one of my favorite Harvest Moon games so far.

Anyway, my question is this: After beating the boss of the first cave, one of the NPCs (specifically Neumann), is really impressed and says that if I could befriend him, I would probably get something real special. So I loaded up to before I killed him and tried again. Yet my Friendship Glove seems to have no effect, even after using all my RP trying to use it on him, despite having three empty spaces in my Monster Hut. Is there some secret to befriending him, or is it impossible to befriend him and Neumann is just trying to drive me crazy?