From The Heretic Knowledge Vault
Female practitioner in the Veracian Church, visible in the story arcs starting here, here, and here. In the second sequence, Winston Maurel's Ensigerum bodyguards Warrel and Sara Amraphel are disguised as nuns, but there's an actual nun (Maurel's assistant) in the picture too. Whether this woman is the same as the one encountered in the Heretic Knowledge Vault is unknown. The nun appearing in the third sequence looks a bit older than the others, and may or may not be a different character.
Nuns apparently have some magical powers. When Ian uses a Flame Volley to create a distraction (if not outright death and destruction) in the Heretic Knowledge Vault, the nun with the NPC Guards counters with a Barrier spell, which avoids the messy problem of fried guard (and nun) but possibly worsens the fire damage done to the books. Later, in the sequence where the Veracian Church compound is under bombardment by the Anilis-possessed version of Ian, a nun casts another Barrier that protects Jeramel and other, unidentified church officials, until Ian cancels out the spell (and, possibly, the nun). In the Maurel sequence, Jon hesitates over his kill to ponder whether one of the "assistants" has healing skills. This gives Sarine just enough time to appear, annoy Jon, and then probably save his life.
In the aftermath of the Maurel affair, Paul points out to Anita that the nun who was part of Maurel's entourage stayed around long enough to be an information source, and is now interested in joining the Ensigerum. Anita's reaction to this seems to be lukewarm, but as she's decided that she needs some agents within the Veracian Church, some accommodation might be getting worked out. Stay tuned.
Whether there is any real difference between a "nun" and a "priest" of the church isn't clear yet, other than of course gender.
Nuns in Errant Road
Several nun characters appear in the Errant Road on-line, free-form role-playing game, notably player character Sister Rose, who is attached to a "Reformed" branch of the Veracian Church that doesn't exist in the Poe-verse. Some of these nuns, including Rose, have serious magical skills, and many have other characteristics that are not known to exist in Errant Story itself. They also tend to be, for lack of a better word, nicer than the battleaxes who bear this sobriquet in Errant Story. They should accordingly be considered non-canon, although they're interesting in the Errant Road context.