The duty of being Prince Lothric’s wet nurse fell to High Priestess Emma. In her dying moments, her Japanese dialogue quickly shifts from calling him just “the Prince” to “Lothric-sama“, a respectful address which nonetheless uses his name. In other words, this loyal subject is showing a degree of intimacy when decorum no longer matters — one of the few to apparently see Lothric as a person and not just the royal son. Her dialogue in general expresses the most concern for his destiny, and she is responsible for blessing the holy sword which he was intended to wield as a firelinking hero. The fact that said blade was never wielded by the prince yet can be derived from his soul requires that he have deep personal experience, meaning that it was likely shown to him — perhaps repeatedly — by Emma. She was the one then swaddling the sickly babe in their old prayer clothes for health. She was the one leading the priests in prayers for the frail child’s recovery. She was the one entertaining the bedridden boy with tales of his glorious future swinging that platinum sword as the world’s savior. As High Priestess, she would nurse him back to health along with just nursing him.

The Prince, Lord Lothric, please… please, do save him. Tell him to please become a King.
In that case, for raising a living ritual sacrifice doomed to die young, Emma acted like the prince’s mother, perhaps more so than his actual mother. But the wet nurse was still the High Priestess of Lothric’s church. She believed in the perpetuation of the Age of Fire, thereby viewed her charge’s sacrifice as necessary for the world’s salvation. Her soft spot for the prince thus didn’t stop her from standing by while the royal family performed all sorts of horrors to make him qualify as a firelinker. She might have personally not approved of these actions considering their source, but she was ultimately a Pillar of support to the king and couldn’t, or rather didn’t, prevent the abuse. In the end, Emma could only sympathize with the prince’s tragic life as his surrogate mother. This put her in an awkward spot when said prince lashed out in the midst of their kingdom falling apart. She understood his position, but his “childish” refusal to be Lord of Cinder put the world at stake. He needed to step up and fulfill his duty to prevent the apocalypse.
These divided loyalties had made the High Priestess play the enabler, redirecting us away from the castle when one of our targets lies inside. In fact, she seems to be a collaborator. From an obscure corner just outside the church, one of the knights in the prince’s guard stands over the bodies of two lower-rank knights he presumably just killed. This is the only one not guarding the prince’s chamber, so he must have a reason for leaving his post to come all the way down to the bottom — such as orders from his prince. Factor in the proximity to Emma, and the knight was most likely sent as a bodyguard, someone who protected her from the Hollows plaguing the area before becoming a Hollow himself. He also doubled as her monitor.
Among the prince’s guard are knights with the greatshield denoting the High Priestess’ personal guard. This suggests that at least two of such holy guards were provided to the coddled prince for his protection. And yet, it wasn’t they who were sent to watch over his precious wet nurse but a regular knight with no ties to her proper guard. Evidently, the Twin Princes didn’t want anyone with dual loyalties so close to Emma. The knight needed the willingness to slay his charge in case the High Priestess tried betraying Prince Lothric to those who would do him harm, though clearly she wouldn’t have regardless. Even still, he exemplifies the tension in the two’s relationship over their irreconcilable positions. The prince is using that bond as leverage when he has no intention of ever listening to his old wet nurse, forcing Emma to straddle a fine line because of it.
When the Outrider Knights came from the Boreal Valley, the High Priestess allowed them passage into the castle to contend with the rebels and Hollows, the old woman’s continued survival proving her lack of resistance to their invasion. But when we encounter her, she gladly points us in Vordt’s direction. This cannot be as their collaborator because she won’t inform us of the knight’s post without further prompting and doesn’t instruct us to show the banner to potentially avoid conflict. The order to escort envoys bearing the flag evidently hasn’t been rescinded by Sulyvahn, so the knight should theoretically grant us safe passage if we just show our credentials. In essence, Emma’s selective silence all but guarantees Vordt’s death by our hand while still taking full advantage of Irithyll’s aid to reach the Undead Settlement below the High Wall. The language of her forewarning likewise conveys disdain for the Pontiff’s watchdog. Why exhibit such open hostility toward the Outrider Knights whom her church called in?
Emma more than likely did begrudgingly agree to cooperate with Sulyvahn’s forces, but she is vehemently against their main objective: eliminating the crux of the current chaos, Prince Lothric. So, she ostensibly plays along as these knights’ ally to avoid their bestial ire but misdirects us away from the prince at the first opportunity. As High Priestess, she knows about the mission given to unkindled in these emergencies and has apparently already received word about the Lords of Cinder leaving for their homelands stagnating around the city. This gives her the perfect excuse to keep us out of the castle, betraying the Outrider Knights when they aren’t looking — or if they are, with a justification for one of theirs’ “sacrifice” to the greater good. This proves to be an obvious attempt to buy time in the hopes that Prince Lothric will change his mind and fulfill his duty.
After we collect our last cinders from a Lord outside Lothric, Emma seems to take notice of the First Flame edging ever closer to darkness thanks to the eclipse and speaks to us via telepathy. At the same time, she performs some holy ritual warping us back to her location. This magical connection may be thanks to the banner she provides. Since it bears the sacred ring symbol and is used by messengers, the flag may have also been imbued with holy magic, enabling the High Priestesses to call the envoys back once the messages are delivered — they can then immediately hear the response. Regardless, Emma teleports us back so that we can bring the last “Lord” to the First Flame, hoping that we will “save” Prince Lothric by convincing him to actually take up the mantle. Had she survived, Emma would have most likely joined us in this final attempt to plead for her surrogate son to see reason. But the Dancer realizes the treachery and cuts the old woman down, leaving her to barely complete the ritual and share her wish before succumbing to the blood loss copiously staining the rug underfoot.

… Oh, fireless ash. The fire increasingly darkens and awaits the last King. The Prince, Lord Lothric, please… please, do save him.
While a dramatic end, it is also possible to kill Emma before this juncture. Even in this scenario, the same cutscenes will play with her voice-over, and we will warp to find the same holy circle and blood stains without Emma, obviously, to generate them. Such oddities scream oversight by the developers. This is fair, considering how rare it is for players to kill an obviously friendly NPC on their first playthrough — for fear of missing out on content if nothing else. Combined with the convenience of instantly reaching the objective, and it is understandable if the narrative implications never came to attention. But either way, Emma’s premature demise still has her thinking of her dear prince and the Kingship he must accept. She would rather he die a hero than a traitor, but the boy she raised doesn’t care. He won’t even let us get a word in before siccing his brother on us. Our presence is proof that the old woman failed to do her part, so there is no more that need be said. That is how little his “mother’s” life means to him now.