Hello everyone, new user here.

I'm working on the preliminaries for a from scratch Only War campaign and I wish to include an Imperial Saint as an NPC. Anyone got any good ideas for where I might happen upon game mechanics? Homebrew suggestions? Anything would be helpful.

Well, I don't think there is necessarily an archetype for an Imperial Saint. A "normal person" should do, or at least someone with higher than normal stats and talents.

If you wanted to make the saint far more epic, I recommend Dark Heresy v.1 sourcebook "Blood of Martyrs" as it contains a big section on Faith based powers generally available to True Faith priests and the Adeptus Sororitas

Ah, okay, I was wondering where to find the Sororitas. Kind of got into the whole thing ass backwards and haven't got any Dark Heresy books yet.

At least check out several of the commander npc profiles under an elite or master type style creation to begin to gauge to at least having a starting point.