Future Press game guide description: "An imposter who replaces the real Iosefka in the evening after you enter the Oedon Chapel for the first time. Though her origins and affiliations are a mystery she is a fanatical mad doctor in the classic sense - she'll initially ask you to send refugees to the clinic on the pretense of providing them with legitimate medical treatment but after sneaking in through the rear entrance accessed by way of the Forbidden Woods, you'll discover her true intentions and force her hand…"
Imposter Iosefka is a special Healing Church doctor and possibly a member of the Choir. She wears the white church attire, which is the attire of special Church doctors. These doctors are superiors to the black preventative hunters, and specialists in experimentally backed blood ministration and the scourge of the beast. They believe that medicine is not a means of treatment but rather a method for research, and that some knowledge can only be obtained by exposing oneself to sickness. This description almost exactly mirrors Imposter Iosefka’s actions.
After the player fights Father Gascoigne, Imposter Iosefka will replace Iosefka at Iosefka’s Clinic. To do so Imposter Iosefka incapacitates the real Iosefka and transforms her into a Celestial Emissary. We find the Iosefka Celestial Emissary standing near the door that she barred the player from earlier in the game and when defeated she will drop Iosefka’s Blood Vial.
Even though Iosefka and her Imposter appear to look exactly the same there are some subtle differences. The first is that the voices are different, and are in fact voiced by two separate actresses. Second is that the Imposter Iosefka will behave like she is meeting the player for the first time, even if they have talked to Iosekfa before. Third is that her personality will change drastically. The real Iosefka under no circumstances will open her door to the player or any patients. She does not ask the player to help her or in fact, do anything for her but rather tries to help the player by giving them specially refined blood. Imposter Iosefka takes the first opportunity she has to ask the player to deliver any survivors to her clinic that they find while out on the hunt, saying she will “look after them, perhaps even cure them.” She even offers rewards as an incentive.
Imposter Iosefka wants these survivors so that she can experiment upon them. The white clad doctors of Church believe that medicine is not a means of treatment but rather a method for research. Which begs the question, what is Iosefka trying to find out? After urging you to send her more patients she will tell you “We must find a way. To surpass our own stupidity”. This sentiment almost exactly echoes the thoughts of Provost Willem from the Eye Rune description:
Disillusioned by the limits of human intellect,
Master Willem looked to beings from higher planes for
guidance, and sought to line his brain with eyes in order to
elevate his thoughts.
And Micolash’s raving dialogue:
"As you once did for the vacuous Rom, grant us eyes, grant us eyes. Plant eyes on our brains, to cleanse our beastly idiocy."
To find a way to elevate humanity, with the end of overcoming the Scourge of the Beast, Imposter Iosefka is experimenting with the patients you send her. She will tell you: “this time, I'll be trying old blood.” giving a window into her research methods, and cheekily remarks: “and cheers, to the discovery of kinship.” after giving you the Blue Elixir. The use of the word kinship is clearly a pun:
Kin Coldblood description:
Coldblood of inhuman kin of the cosmos,
brethren of the Great Ones.
Use to gain unspeakable Blood Echoes.
Dare not to delve into the world beyond humanity, the
eldritch Truth touched upon long ago at Byrgenwerth.
Imposter Iosefka’s work is closely mirroring that of the Choir. The Choir are an institution of the Healing Church made up of the highest-ranking clerics and scholars who continue the work began at Byrenwerth. The initial encounter with Ebrietas in the tombs below Yharnam marked the start of an inquiry into the cosmos, and led to the establishment of the Choir. The Choir believe that the sky and cosmos are one, together with the left behind Great One (Ebrietas) they look to the skies in search of astral signs that may lead them to the rediscovery of true greatness. This description implies that they have discovered greatness already, and is further implying they search for more Great Ones (especially with the pun on “greatness”).
To do this the Choir use the blood of Ebrietas to create “potent unseen thinkers” - Celestial Emissaries - who are able to reach out to the Great Ones. These Emissaries are not Great Ones themselves, they are kin of the Great Ones. We know for certain that Iosefka is creating these Emissaries in the clinic because once created, the emissaries behave like they did before they were transformed. For example, the Iosefka emissary stands idly behind the door, and the Afflicted Beggar emissary will kneel on the ground scoffing up apparently nothing, mirroring the pose we find him in when first encountered. But even more telling than this is the body left on a table at the back of the clinic. What it shows is a partially transformed Celestial Emissary, but the left hand and the left upper thigh remain human.
Imposter Iosefka does not dress as a member of the Choir, but she does use their secret rites. A Call Beyond is one of the secret rites of the Choir, only known to a few characters in game like the Choir member at Byrgenwerth and Micolash. Iosefka is also creating the creatures we find en masse in the Upper Cathedral Ward, the home of the Choir.
But if Imposter Iosefka is following in the footsteps of Master Willem, why does she use blood? This is perhaps where the paths of Master Willem and the Choir diverged as is stated in the Blindfold Cap description, since we know that Willem would not have anything to do with blood, considering any who would use it to be a traitor to him.
While we can guess with some level of certainty at Imposter Iosefka’s intentions, it is less clear on how she intends to reach her goals. She seems to be trying anything she can possibly think of on as many subjects as she can get her hands on. To Imposter Iosefka finding a way for humans to ascend and curing the Scourge of the Beast are one and the same.
But none of her experiments work as she might have hoped. The Celestial Emissaries are kin, and certainly not beasts, but they are mindlessly aggressive, hostile to any who encounter them. By the time the Blood Moon rises Iosefka is forced to admit that there can be no survivors out on the streets who haven’t turned into beasts, and therefore no one left for her to experiment on. The white clad doctors of the church believe that some knowledge can only be obtained by exposing oneself to sickness, so with no one left to experiment on, Imposter Iosefka experiments on herself. But this time she doesn’t necessarily use Old Blood, she consumes a One Third Umbilical Cord. From the description:
A great relic, also known as the Cord of the Eye. Every
infant Great One has this precursor to the umbilical cord.
Provost Willem sought the Cord in order to elevate his being
and thoughts to those of a Great One, by lining his brain
with eyes. The only choice, he knew, if man were to ever
match Their greatness.
When we encounter her we find her prostrate on one of the operating tables, twitching uncontrollably. Her dialogue clearly indicates she has eyes ‘writhing’ in her head and that it is an experience both nauseatingly painful and “rapturous”. The eyes are allowing her to see new things. She will tell us: “I knew it, I’m different. I’m no beast… I…” Despite her apparently sociopathic behaviour and cold demeanour, Imposter Iosefka appears to be well intentioned. It is just her means to an end that is lacking in scruples, and actually follows the methods of the Healing Church, seen even from its early days at Byrgenwerth, through to the Research Hall victims, the actions of the School of Mensis and Choir. These institutions would sacrifice anything to reach their goals, seemingly finding little to no worth in humanity in its current form.