I might be stating the obvious, but since the game themes are about desires, i think that the costumes, expecially in the phantom idols' case, are a manifestations of what they want instead of their rebelion. Okyann misses when she was a fashion icon, Rin misses her culture, Mont misses being an ice skater champion. The new girl (dont remember the name) wants to be a nurse, Reo wants to be an actor. Riddle probabily wants to be a detective when she grows up instead of a model, Puppet a marine biologist etc etc. But in some cases it can be a bit confusing. Im not sure what Seji or Seppia's costume could be. maybe Seiji its a butler because he wants to serve instead of being served? Seppia's homeless so maybe he wants to have a lavish life? Idk im just rambling rn
Honestly, I think it's really just a mix of both. Okyann might miss being a gyaru, but you could easily also tie it to her sense rebellion- besides herself mentioning in her Confidant that she tends to do things her own way, the gyaru subculture was born from a rebellion against "Japanese social and aesthetic standards" (quoted from Wikipedia). So there's an inherent aspect of rebellion to dressing like a gyaru. It might be that her strongest point of rebellion was just in the past, so her outfit reflects her gyaru days, as that's rebellion to her.
Similarly, with Rin- sure, she misses her culture I'm sure, but you could also consider it part of her rebellion against society that she's sticking to her overtly Chinese culture rather than, you know, moving to Japan for school and then conforming to Japanese society wholeheartedly. Holding onto that is her rebellion.
Leon and Marian (that's the nurse girl) both have something they're aspiring to be, despite other forces (Leon's parents, Marian's sense of responsibility to her family's store) pushing them to do something else. Probably Puppet, too, since she ran away from home, so I bet she wanted to be something (presumably related to the ocean) other than what her parents wanted her to be. Being these things in their Metaverse outfits is their rebellion.
I'll admit Riddle, Mont, Fleuret, and Sepia are a little trickier, but in large part because I have less to go on about them at the moment. Riddle's been set up by her video as kind of a smart kid for her age, and her outfit seems to be some kind of miner/inventor, so it might be her using her intelligence + curiosity to do things that adults think she can't? Mont's rebellion could be that she wants to keep skating despite the injury that stopped her, or, you're right, it could just be her desire to go back to when she could skate. Fleuret reminds me of a revolutionary, around the era of the Revolutionary War and the French Revolution, which has inherent ties to rebellion, but I don't know enough about him in the real world to speculate exactly what it means to him. Same with Sepia, except I'm not entirely sure what his outfit is to begin with, hah.
All that to say, I think there's just a strong correlation here between rebellion and desires, haha! To be clear, I don't think you're wrong, I think it's just typically a mix of both here.
Fun news: The start of Kayo's Confidant actually confirms she and Reo are related, though I think her dialogue might be implying they're relatives in law, and Reo's technically directly related to her Darling (husband)?
From Google Translate, dialogue during the interaction with her that establishes her Confidant:
Kayo:
"Ah, I'm sorry, it feels strange to say this suddenly, right?"
"... This is how it is, there is a child from a relative who is coming to stay at my house recently."
"That kid graduated from high school this year and should be about the same age as little Wonder, right?"
"I heard that he has a dream to become... I remember it is... What is the special photography actor called?"
"When he came to Tokyo, he seemed to plan to work part-time to earn tuition while attending an acting school."
Wonder:
"Very impressive"/"It will feel very hard"
Kayo:
"Right?"
"In an era like this, as an adult, of course I want to cheer for him to be able to leave his parents and walk alone with his dreams in mind."
"However, it is too difficult for young children today to grasp the sense of distance, right?"
[skipping past a little tangent of hers for the sake of this post]
... "If I put pressure on the kid, it would be harder to say something to cheer him up, right?"
"We also have to consider the relationship between relatives. I don't want to cause trouble for Darling because of myself."
That last line is what makes me think he might be the son of a relative specifically on her husband's side of the family, since otherwise it seems a bit odd she's worried about potentially causing trouble between Reo's parents and her husband. But it's not impossible Reo's actually on her side of the family, and either way she evidently does consider him part of her family.
(As an aside, later in this scene she also mentions her Darling works in the fashion industry, and she herself used to be a fashion stylist, so it's cute they have that shared interest!)
It seems that in the Spotlight video, Reo refers to Kayo as 阿姨, making them nephew and aunt. If that’s true, I want to know more about their relationship….!!!!
Uhh, maybe! I hate to be a downer here, but I know it's common in a lot of languages (such as Japanese, and I believe Chinese as well, although I'm less familiar with it) to refer to older women as "aunt" or "grandma" even if you're not related to them. It's a respect/politeness thing!
So, unfortunately, I think it's a lot more likely he was just being polite/at least friendly, rather than them actually being related. But it's not impossible?
I've been wondering about this ever since I saw Okyann's and Puppet's All-Out Attack finishers, how come in P5R, futuba doesn't wear her mask in her All-Out Attack Finisher since she's the navigator and she's always inside her persona, but Okyann and Puppet both have their masks on when they do their All-Out Attacks, simulating that they have to be the navigator which means they would have their masks on.
I'm honestly not sure! The circumstances between the three of them are very similar- all three navigators sit/stand inside of their Personas in order to navigate- so wearing their masks would seem to suggest Puppet and Okyann no longer have their Personas summoned during their All Out Attack finishers, despite the fact that Oracle seems to still have Necronomicon summoned during hers, since she doesn't have her mask. It's a little weird.
Then again, Futaba is kind of the odd one out among all the Thieves. She's the only one in P5/P5R to not be wearing her mask during her AOA finisher, and in P5X this continues to be the case so far. It honestly might just be that she's the exception, not the rule, here.