Design similarities with Cure Eclair Candidates. I actually think Cure Eclair might be all four.
Shared Motifs
Eriza (author) and Kurea (patissier) share the buttefly (ps not circled but butterflies are also on eclair's shoes), while Rei (student council) and Shiruku (actor) share the star motif (shiruku's star is a different colour though).
Shared Hairstyles
Eclair's curls in her ponytail are similar to Eriza, Rei shares hair whisps on the fringe, Shiruku has the hair loops, and Kurea (as well as Eriza and Rei)have loose hair from the sides of the face (not circled in the image for Eriza)
Shared Colours
Shiruku has the closest colours to Eclair as both of their primary colour is light blue.
Rei's dark turquoise hair could transform into blue, plus the green fade at the bottom.
Kurea also has dark blue hair that fits Eclair's colours.
Eriza has a green dress which matches with the fade at the end of Eclair's ponytail.
There's also purple/pinkish colours from Rei and Kurea which can be seen on the underside of Eclair's overskirt.
Other
All four of these characters are listed side by side on the official toei website in the extra character section (top row as well).
Personal Theory
I saw someone say that these characters are all Cure Eclair at the same time, which I initially just passed off as a crack theory. But I thought about it. With the Space Time fairy and the whole time-travel shenanigan, something like this isn't out of the question at all, perhaps there are multiple manifestations of Cure Eclair. It would also make sense to keep Cure Eclair's identity a secret and create so many potential candidates for her if they went this route. It could be passed off as intentional red herrings, but all of them seem to share similar amounts of design similarities.
But if it is only one person, I hope its Rei she's my favourite :)
WAIT. THE MAKOTO JEWEL SHATTERED. ECLAIR SHATTERED AS WELL. OH MY GOD THEY REALLY ALL ARE ECLAIR. THEY ALL HOLD A PIECE OF ECLAIR. THATS WHY WE HAVENT SEEN ECLAIR, SHES SPLIT APART INTO 4 PIECES. (this has the potential to be the craziest transformation sequence ever.)
(and then it turns out to be nijee)