I would strongly recommend reading the SR novel "Burning Bright", which has an ally spirit as one of the key supporting characters.
The ally's personality and all depends a lot on the summoner, i.e. the character and thus the player. One of the key components of the ally spirit, however, is to provide a sounding board for the character to bounce ideas off. This can make the ally generally pleasant (such as Harry Dresden's Bob) or something of a pain in the ass (the equivalent of a Devil's Advocate and general downer kind of guy). It could be obscure, it could be direct; it could even be painfully direct about some things, even if it remains relatively discrete. It should remain generally helpful, though - even though sometimes the help it gives is help that the character doesn't want, but still needs.
I prefer to have it NOT have curiosity as its primary personality element; instead, consider the PC's characterization over the past games, then go over to 'free spirits' and decide now what kind of free spirit it would be should it ever break away from its summoner and yet decide to stick around. I personally prefer 'player' - which IMO is what Buttercup is, because when you get down to it, she sure as hell isn't an Anima - because it allows for real interaction, instead of an eager-genie-like 'Yes, Master, right away!!!!11!oneone!!1!'
In addition, eyeball the spirit's Force, and thus its level of intellect, etc. Consider that at the time of its summoning, it may very well know essentially everything that its summoner did, plus other odds and ends as well; anything over a Force of 2 (meaning anything useful) is going to have its own ideas, thoughts, goals, and ambitions.