Eriza as eclair and Kurea as shushutan theory
IDK if this is the most likely theory to me but i feel like its another way to interpret the hints we have so far.
I do think that Kurea is 100% involved either as eclair herself or shushutan. I also think that there are some pretty suspicious things around Eriza both in the story but also in the shows directing.
1- Kurea as Shushutan:
The first hint to this is the scrunchie thing. Ive seen the theory that shushutan is hidden as the scrunchie in Kurea's hair but weve never seen a fairy in this show transform into an object, only into people. Id take the scrunchie thing as a hint that kurea would be shushutan. Another design aspect is kurea being the only one of the 4 to have pink/red in her design (not countng rei's uniform) which matches with the bow shushutan has.
Second is that the only real thing we know about shushutan is that she is very knowledgeable. I think that so far Kurea has contributed not through much sleuthing but more through giving suggestions based on her own knowledge. Some examples are informing that fragrance is linked to memory, the meaning of lilies and her own sort of inherent knowledge of things.
We have 0 knowledge about Kurea beyond her working at the bakery which would allow for her to be a fairy secretly and only being hired at the bakery relatively recently. On the other hand the other 3 eclair candidates have family, close connections or enough of a past to make her being a fairly less likely.
Kurea has an almost supernatural ability to just know things based on inner truths like the cake Anna wanted or the hunch of where the fight was happening. While it could just be Kurea having good intuition it could be related to shushutan's actual powers. Only fairies have used magic outside of cure form so far.
This feels like a potential way to explain why luluka was so strange towards kurea while also allowing Toei to throw us off with the mistery.
2- Eriza as Eclair:
First hint towards this is Eriza's history as a writer. In the 2 episodes focused on her she joins the girls to solve a mistery and is inspired to write a novel about it. However we know that before the show's story begins she had already written a very succesfull novel. From her episodes we see that she relies a lot on hands on experience to be able to get ideas for both writing novels and solving misteries. I think this points to her having had some prior experience with detective work which led to her writing her first novel. She could have just done it without any direct inspiration but it feels less likely.
Second she is the candidate who so far has done the most sleuthing. I also think she would pair well with a knowledgeable fairy like shushutan and their skills would compliment each other in the sleuthing. Eriza's method of taking on the role of what she is investigating also matches the tanuki thing of transformations. If kurea was shushutan she would also have a contrasting personality to eriza in a similar way to luluka and mashutan.
The eriza focus episode (ep 21) has had the least involvement with arcana shadow. In fact this episode actively goes out of its way to ensure that luluka does not see eriza despite both being at the agency at the same time. There is this shot while the girls are showing Eriza the rooms in while luluka is standing outside. We see her looking in and spotting the manager.
The shot very intentionally shows how Luluka cant see the other three girls inside due to the curtain and mashutan even comments how its in the way afterwards. This feels very intentional to me. If it didnt matter at all that luluka saw eriza at the agency dressed as a detective then a more normal way to do this shot is to have luluka behind the wall looking into the whole room. Though maybe its just so mashutan comments on the curtains not being stylish but hiding the other three girls is still odd.
While luluka did briefly see eriza in episode 16 I could see her lack of reaction being due to her not thinking Eriza had no hope of being helped at the time. Maybe seeing her in the rooms at the agency dressed in a way clearly resembling eclair would make her react in a suspicious way. Eriza also directly reacts to the picture of the blue butterfly on the wall more than Kurea did.
I think this is a flimsier theory than my Kurea as shushutan theory but it feels possible. I kinda hope its true because i find eriza to be very fun.
I do still think its likely that Kurea is Eclair however I also think that the story is being a little too obvious with it and a misdirection with kurea would be a good way to make the reveal unexpected and fun. Having such a good time with this arc and maybe next week's episode changes my theory entirely.