well, perhaps as assassins get better (increase in valour), this is reflected in the game by having the monkey acquire some skills. Or perhaps the monkey gets special attributes with practice, thus increasing the assassin's overall efficiency.

I guess one could imagine a monkey being able to steal stuff, or replace small objects, or crawl through small openings and pouring poison into a glass, etc. After all, they can be trained.

If none of the above, well, at least it's funny