NIjee

I know its incredibly unlikely since there's far more evidence for the four main candidates (Shiruku, Kurea, Rei, and Eliza) to be Cure Eclair, but as someone who really likes Nijee (I haven't liked a precure villain this much since Julio from KiraKira), I would be so happy to see him as the villain-turned-Cure.

My (Weak) Evidence

While its obviously not as compelling as the main four candidates, Nijee's hair (particularly the side bangs) does have a vague resemblance to Cure Eclair's hair and his star shaped pupils and facial mark look similar to the star shaped patterns on Eclair's hair and outfit.

Out of 15 episodes so far, Nijee has been the villain to do the main thieving and summoning the monster of the week for 7 of those episodes. Meanwhile both Ageseine and Goemon have done the thieving and summoning 4 times each on the other hand.

I find it interesting that by Episode 5, even though Nijee had only failed twice, Usonoir was ready to drop him. But Ageseine and Goemon went on to fail multiple times themselves and they have never been threatened like Nijee was. Heck, Cure Arcana Shadow openly defied Usonoir by getting in his way in Eouside 13 and yet was never punished for it. In my opinion, Usonoir has it out for NIjee more than his other minions

Nijee's distress being openly shown in Episode 6 is also interesting as none of the other villain barring Arcana Shadow) have been shown in that manner. Overall, I'd say that Nijee is slightly more fleshed out than his two cohorts, again not counting Arcana Shadow

In Episode 15, when we see the flashback of Eclair (who is not fully shown) shattering the Makoto Jewel, only Ageseine and Usonoir are present, not Nijee or Goemon, which granted, could mean nothing, but still worth pointing out.

Usonoir has the ability to brainwash as we saw in Episode 13, so is it possible that Cure Eclair didn't die or shatter after what happened with the Makoto Jewel. Perhaps Usonoir brainwashed her... or him in this case. And perhaps Luluka, after finding out what happened to Eclair, infiltrated the Phantom Thief Guild to investigate further.

It's fairly obvious that there's a connection between Arcana Shadow and Eclair. In Episode 12, it's heavily implied that she had someone dear to her in the past and in the flashback in Episode 15, she's shown watching helplessly as Usonoir was about to take the Jewel until Eclair came to stop him and she watches in horror.

While we don't see much interaction between Luluka and Nijee throughout these 15 episodes, and she seems as indifferent to him as Ageseine, I'm intrigued that during Nijee's crisis in Episode 6, she was present with him. Yes, she seems indifferent, but Luluka clearly keeps her emotions subdued. We don't know what she actually feels during most given situations.

Whether, in regards to this theory, Luluka outright knows or only suspects that Nijee was once Cure Eclair, she can't make it known and keeps her poker face as part of the act. She doesn't make it obvious that she is specifically looking out for Nijee to keep Usonoir from getting suspicious.

As for Usonoir's attitude towards Nijee, especially in Episodes 5-6, it's possible that he was disappointed in Nijee because he thought that a Cure-turned-villian would be a powerful ally, only for Nijee to be continuously a failure. And who knows, the mysterious room under the trap door may not have meant death for Nijee, but rather something else. Maybe Usonoir, after "disposing" of Nijee, actually planned to do something to him in secret (brainwashing him more or giving him more power or something)

Now, I'm not gonna lie, there are many holes in my "Nijee is Cure Eclair" theory, too much to go over so in this already long post (I might make a Part 2 if this post gets at least some attention). At the end of the day, I do think its more likely that Cure Eclair is one of the four main candidates or perhaps even all of them

But hey, this is a season about mysteries and solving them, so I just had to get this theory out there, even it literally gets debunked in the coming episodes. I'd like to know your guys thoughts as well

Also, I haven't caught up with toy leaks, so I don't know if they contradict this theory already