After Season 4’s first episode, I believe there’s a very good reason to believe this. Episode 67 hints that Foxidna very much remembers Julius even though he had his name eaten by Roy Alphard, one of the Sin Archbishops of Gluttony when the show previously led viewers to believe that Subaru was the only one who could remember such a victim when everybody else forgot, and conveniently, Foxidna also puts it into Subaru’s mind that it’d be a good idea to take Rem with him to the Pleiades Watchtower so that he could have the perk as being the first one she sees upon being cured, which is showing the same sort of manipulation skills as the original Echidna and Tea Party Echidna.
As a refresher to those who don’t remember all my theories or to segue those who have never read my theories before into the meat of my post, there is a basis to believe this given that everything up until Season 2 was planned at least partially by Echidna centuries ago and probably by Flugel too given he made Petelgeuse promise to not take in the Sloth Witch Factor, which was implied by Petelgeuse apologizing to Flugel, most likely so that Petelgeuse wouldn’t until Pandora and Regulus invaded Elior Forest when he had no other choice to protect the love of his life, Fortuna, and Emilia. A similar event involves how a special font was used in the Frozen Bond OVA to denote a female’s speech regarding a contract with Puck in which he would lose his memories and perhaps other aspects of himself if he entered into a contract with Emilia with that female’s speech being revealed to be that of Echidna based on Season 2 reusing that font. That both Echidna and Flugel both made someone promise not to do something only for them to do it when there was no other choice with Regulus and Pandora’s arrival for Petelgeuse and Melakuera’s assault against Emilia for Puck may indicate that one of Echidna and Flugel learned that sort of behaviour from the other, meaning there was an age difference between them. Alternatively, they could’ve just closely collaborated on things like I have speculated, but these things aren’t necessarily mutually exclusive.
As Season 2 revealed through Roswaal and Season 3 reiterated, Echidna’s not-quite-Tome of Wisdom, however many there are given Echidna might’ve lied about how many copies she made, is the prototype for the Witch Cult Gospels, meaning that they are most likely at least partially made by some of the same materials, and given how Roswaal’s not-quite-Tome of Wisdom was not meant to bring about the return of the Echidna Roswaal knew, the same could go for Gospels in not directing their holders to their goals. I don’t buy into the theory that the reason Otto wasn’t in Roswaal’s not-quite-Tome of Wisdom is ’cause he was meant to have died as Ram, Petra, and Emilia all died in Arc 3, and with that being the case, it can be argued that all three of them were meant to have died, so the true reason Otto wasn’t in the not-quite-Tome of Wisdom must have been something else. As a text that supposedly tells the future and can update itself based on current happenings, there’s no good reason why Otto was excluded unless the source of the knowledge of the Tome just didn’t want it to be included, and if that’s the case, Echidna may very well have been responsible for its contents with the common denominator between the not-quite-Tome of Wisdom and the Witch Cult Gospels possibly being that they are made at least partially from a body part or body fluid of hers much like her tea in the tea parties with these texts being connected by magic to her mind/soul since we know the soul is the seat of the mind in Re:Zero, which is why Subaru remembers his past lives even after Returning by Death ’cause his whole body resets to a prior point, but his mind/soul doesn’t.
This implies that the Echidna from four centuries ago, Tea Party Echidna (mostly her), Foxidna, Flugel, and others from that era were driving events from that timeframe until present-day Re:Zero with a common future in mind with examples being Puck being in a contract or oath with Echidna to not enter into a contract with Emilia or lose some of his memories should he choose to, Flugel conveniently planting the world tree known as Flugel’s Tree in the exact spot the White Whale extermination force needed to trap the White Whale’s main body, Petelgeuse apologizing to Flugel in making his decision to take in the Sloth Witch Factor, Roswaal saving Ram and Rem after allowing for their village to be wiped out, and the White Whale leaving Subaru alone in Episode 17 through someone’s order, probably Pandora’s, even though there appeared to be orders to not allow people to inferfere with Petelgeuse’s Ordeal to Emilia, meaning Subaru was allowed to escape from the White Whale so that he could learn about the White Whale’s Fog of Elimination, inadvertently kill Emilia, pushing him to the point of giving up, and see Petelgeuse’s Authority of Sloth: Unseen Hand, so that Rem could give him his From Zero moment and so that Subaru could plan his counterattack against the Witch Cult.
One big thing about mabeasts that we learned in Season 2 is that they were created by the Witch of Gluttony and that they find the scent of Satella and Subaru, irresistibly delicious, showing it’s probably the miasma of the Envy Witch Factor or the Envy Witch Factor itself that is irresistible to them. Thus, given that Subaru being pushed to his breaking point and finding out about Petelgeuse’s power was all conditional on the White Whale chasing after him, that the defeat of the White Whale was also conditional on Subaru being irresistible to the White Whale, and the defeat of the Great Rabbit was also conditional on Subaru being irresistible to that mass of rabbits, Daphne, the previous Witch of Gluttony, and her mabeasts might have specifically been drawn to one aspect of the Envy Witch Factor that I talked about, either partly due to experimentation (from herself or somebody else) given the weird coffin and restraints associated with her along with the weird shape of her arms, which perhaps suggests she is far more than just a normal human with the power of a Witch and that there’s more to the story of her true nature. If I’m right about this, Daphne may be a homunculus of sorts made specifically as part of an overarching grand plan from the group of heroes from four hundreds ago to bring the world to a free future, and she may be the first the host of a Witch Factor given the signs of experimentation on her that aren’t present on the other dead Witches of Sin from Season 2. Now I’ll have to take a detour before getting back to this point.
In my main theory, Satella and the hero group, which includes Satella and Flugel, from four hundred years ago started a grand plan to cause an intervention that would end the cycle of the world being constantly destroyed and reconstructed by forcibly ensuring that the Hero of the story at some point would be weak enough, as in Subaru, since that’d be the best way for the Hero to overcome his character flaws unlike the past Heroes of the story who I posit had everything come easy to them, and that means the heroes are the reason for the Witch Cult’s founding and even why Pandora took over the Witch Cult eventually, and yes, I do consider Pandora to be one of the heroes.
I expect the final boss to be someone with an Authority that allows him/her to control the story path. This would make Melakuera stating that he faced world-ending crises countless times makes sense if the world was continuously being destroyed and reconstructed before the age of Satella. Melakuera remarking about the world imbalance and Witch suggests that the world’s repeatedly destroyed by the Witch of Envy, likely due to the actions of selfish and overpowered heroes of the story of each iteration of the world who choose their women over the world due to never overcoming their character flaws with Subaru saying in Episode 29 only someone who hates the world would choose mayonnaise over the world being foreshadowing of the choice he’ll have to make.
If Satella actually had a contract in which she shares her field of vision with Echidna through a soul, similar to how Satella can read Subaru’s thoughts and can take action against Subaru for doing so, which would explain how Satella knows what should be Subaru’s save points given it’s unlikely for her to remember the exact details from four hundred years ago of the grand plan, meaning she should be getting direction from someone else. As a reminder why it’s literally impossible for Satella to be future Emilia, Satella told Subaru what he gave to her in Episode 38, and Episodes 18, 25, and 38, respectively show that Subaru has dormant memories of Satella given that Puck revealed to Petelgeuse in Episode 18 that if he wants to kill him he needs to summon a thousand shadow hands, half of what Satella could (Half of 1,000 is 2,000), that Subaru used a number associated with Satella to express his affection for Emilia when he said that if Emily names ten things she hates about herself, he’d list two thousand things he loves about her, and after Satella requested to Subaru in Episode 38 that after he comes to love himself that he should one day come kill her, he suddenly had a positive upwelling of emotion for Satella and swore that he’d save her even though he had called her a monster he couldn’t understand shortly before that, which indicates that they used to have a close relationship and were probably lovers. Given Subaru literally has dormant memories of her, it therefore makes it 100% impossible for Satella to be future Emilia, so Emilia and Satella’s relationship is something different than that. This would explain how Subaru hasn’t been trapped in a bad loop that is inescapable, and that’d be due to Satella’s direct collaboration with Echidna.
Thus, I theorize that Daphne may have been part of, as a test subject, in the initial research from the group of heroes from four hundred years ago on how to create Witch Factors, and this is both why mabeasts and Daphne are attracted to Subaru’s scent and why both Subaru and Foxidna are protected from the aftereffects of Gluttony eating somebody’s name or both that person’s name and memories either due to having a Witch Factor or having a high amount of miasma because it was important for story purposes that Subaru be chased by mabeasts and also be able to remember Gluttony victims such as Rem and for Foxidna to also be able to remember those victims in order for Subaru to overcome the challenges he must face and to keep the world on the path the group from four hundred years ago is trying to bring about in which the world is saved and stops getting destroyed.
All of Arcs 1-5 so far (Seasons 1-3) have all developed Subaru’s character in a certain fashion with Arcs 4 and 5 (Seasons 2 and 3) being Subaru’s greedy arc where he tries to reject the type of person Roswaal tried to fashion him to be, and Emilia as well as other characters, whether they be part of the Emilia camp or not, have been developed in ways that may be crucial to the story with Foxidna being perhaps an important driver of this arc’s events in continuing the grand plan in suggesting that the group visit Shaula and for Subaru to take Rem with him with her selling the latter idea very well to Subaru, and I believe Subaru’s fatal character flaw will be dealt with by the end of the story so that he can bring about a future free from determinism.