I'm toying with idea of introducing a living Saint or emphasize the positive Faith angle of WH40K as presented in the Gaunt's Ghost series as a counterbalance to the typical grim-dark style. I'm curious as to how other GMs might handle those concepts and aspects.

The classic approach is the virginal young girl, as shown by in the Ghosts novels. I suspect it owes a lot to Joan of Arc.

Personally Id avoid it cos its cliched. Perhaps a dalai lama style young boy? Or a more mature seer type male character. Sort of Obiwan kenobi meets muhammad? That might work.

Or alternativly, going down the child approach why not have a living saint as a group of spooky precocious kids. Sort of a goody two-shoes Midwich Cuckoos? that might work.

Obscure bit of knowledge for you...

Look up the word: Kumari

Great for eaither a living Saint or a corrupted vassal playing at innocence...

I would agree that if you want to add a Living Saint to your campaign, think about something that is not cliche.

As long as the Saint embodies the will, tenants, and everything else that it means to be part of the Emperor (or related to), that's all that counts. Being virginal, "pure", holy, or otherwise put into a typical "Saintly" mold is not always needed. In fact most people called "saints" IRL and in fiction (including some notables from 40K fiction) were never signs of perfection. It's only when they are shown something greater, and/or awaken something greater in themselves when they change into and become something more than they are. Even then they may not be very much different than they were before, except that they have touched greatness and brought it into their lives. Then again they may be something completely different.

Also being a Saint doesn't mean to be a great figurehead, nor do said people go out of their way to show and push an idea in regards to their higher connection. Some do this. However many are saints are just people that are there doing their own thing that may be influenced by something higher. It's others that usually pick up on this greatness and elevate the "saint" to status. A grungy, drunk, womanizing veteran Imperial Guardsman NCO who inspires and protects his fellow troops can be as much of a saint as a preened Imperial Sanctioned figurehead from an organization who came from greatness and is entirely "virtous" by right and grace.

This thought idea can be used in interesting ways in a game. If players are going around trying to find someone who embodies the typical "saint", they may totally end up egnoring and may never try and find the true saint they are looking for.

In short a "Saint" is not always someone who is "pure" they are someone who acts and inspires.

I would always leave PCs guessin whether or not a saint is a real saint. Is this NPC they meet really holy, or is (s)he an impostor, insane, possessed or even just regarded a saint by a superstitious, fanatically religious populace totally against his or her will?

Most saints mentioned in the fluff are shrouded in mystery and legend. No one knows what really happened to Drusus and how he was resurrected. It is quite possible that saints do not even exist, but were 'created' by the Ecclesiarchy.

I feel this level of uncertainty is needed to portray saints and keep them something of an enigma. You don't want to create the Saint career path