Well, that demon would be Satan himself, and what's the point of lording over Hell if you don't have minions?
As somebody who plays Divine characters almost exclusively, I'll tell you I've only ever played one character with True Faith to any degree of success. This despite the fact that True Faith is one of the most useful Divine Virtues in the game, giving you Magic Resistance, effectively self-replenishing Confidence, and free access to a Divine tradition. Why? Because if it's being treated with the appropriate amount of respect, True Faith is a really hard thing to gain and hold on to. Roleplaying the intense, all-encompassing, other-personality-trait-destroying piety is part and parcel of the concept, but the limits to character actions and amount of stressful attention is beyond compare.
An only slightly exaggerated account of how True Faith should work by paradigm/canon:
"Alright, I'm just gonna take a walk down to the market to buy some fruit." "Why do you have money? Couldn't that money go somewhere better, like donation to charity? In fact, shouldn't you be assisting the sick right now? While you were thinking about it, two of the Princes of Hell snuck up on you. Roll Initiative... Or let them take the first free shots, because there's a tree falling on a helpless kid over there. And another kid is pickpocketing that money you were going to buy fruit with." "Oh, so it's Tuesday?"
I don't generally take as picky an attitude toward True Faith as that, in part because it's not really supported by the material to the extent that you say it is. Saints generally don't act so all-encompassingly pious - even Francis of Assisi doesn't, and he really is faithful to the point of nuts (though part of that is more the fact that Francis is excessively spiritual and, despite having Divine Unity, acts really Gloomed-out a lot of the time).
Still, it was very funny when my True Faith Holy Maga was accused of avarice because of her shabby dress and Spartan lifestyle (because most of her stipend went to the poor), and of course when an overly-zealous Quaesitor bitched her out for constantly stopping to alleviate the diseases of peasants, not to mention her poor consors having to make sure she at least ate a meal a day. But she wasn't a complete idiot; she knew that she should save fatiguing healing rituals for special cases, when a 40-day Recovery bonus works just as well for most problems and can be mass-produced, and likewise she could be convinced to not actively court martyrdom at the hands of the Quaesitores. (Of course, the troupe and I knew that she was still headed on a collision course with destiny, because Compassion as a Major Flaw is supposed to screw the character over, and at some point she'd have to decide between God's demands and the Order's.)