So I'm not sure if this has been brought up at all on this forum, buuuuuttttttttt I'm gonna talk about it because frankly, I've thought about it for a long while, since e16 I think?
Well anyways, one thing I noticed was the owners of the keys matched- color wise- with their respective key.
Anna has purple as her general color and carries the amethyst pendant.
Heather owns the ruby ring, and while she isn't red, pink is a tint of red, so technically she fits the bill. Plus in her battle Sprite, her hair looks more reddish? Not sure about why that is.
Now, for the sapphire bracelet, while the original owners ARE Saphira, Charlotte, and Laura; but seeing as how they wounded up loosing it in a fire, only for Titania to give it to Amaria, I think Amaria is the 'Official Owner'. The color scheme says all basically, but it does bring up an interesting idea: that there's some outer force in the game that already chosen the keys "protectors", which could explain the color coding as well.
But there's one that threw me off, and that's the emerald brooch. What we do know of the brooch is that it came into El's possession by Solaris, and gave him his sight back somehow, and Luna made off with the brooch and claimed Randomus and Sierra as her parents.(E18 potential spoilers under cut)
With the above said, El isn't the brooch's "Owner". Luna is a maybe, but not very likely. But listen:
After Obtaining the Brooch, El gets practically mania and becomes extremely religious, to the point of being a sacrificial cult leader. He's obessive.
After running from El, denouncing him as her father, and stealing the Brooch, Luna claims Sierra and Randomus and her parents, and her family values, especially in the latest episode, are very much highlighted upon, as if family is her top priority.
A trend with this brooch is a strange obessive quality comes to the people who own it.
So here's what I think:
To know who the real "Owner" of the brooch is, they should/most likely possess these qualities:
-color pallet consisting of greens mostly
-obsessive over something (it's past two owners obsessions have something to do with two, incuding the owner, or more people involved with some sort of "togetherness/organization"; a common ground in a way.)
Annnnddd we are down to three candidates.
Lin, Florina, and Fern.
Now, the color thing is self explanatory, so let's talk about the obsessions side of the deal, starting with Fern.
Fern isn't likely, but more likely in my eyes than Florina, but he does kinda have some lead way I guess, so I'll give the benefit of the doubt.
Fern is obsessed with kicking your ass from the get-go it seems. Honestly he's just obsessed with you in general. After no show for a long ass time, he comes back from nowhere to fight you in route 1, then later JOINS TEAM METEOR JUST TO SPITE YOU FURTHER. This kid is the definition of petty.
The only issue with this criteria, but also could be a standing point for it, is that there's no "organization" or togetherness between Fern and the player. How this could be a standing point is that we aren't sure if the brooch will affect the "Owner's" differently compared to those who had it beforehand. It could very well be that the true owner will rupture a disconnect with organizations and/or togetherness between others or themselves. Also the alternative timeline thing that the Beryl ward witch lady brings up. In a word where you don't exist, Fern is the one to take up arms, and that's your togetherness. You both are different sides of the same coin.
Florina doesn't have much going on in her case. I could argue that she's obsessive with being emotionally cold and/or finding her brother, but it's not as pressing or as notable as Ferns rivalry with you and Lin's obsession with power. That's why I don't think she's a likely canidate.
Now Lin. Lin is probably the most likely option of the trio. It's only in reborn's style to make the evil OP lady also be the true owner of the brooch. And not to mention her obsession with power. Like I said with Fern, maybe the brooch affect the true owner differently, by making then against the idea of togetherness, like Lin. She's the epitome of rugged individualism and teases those who rely on each other. Also the brooch has shown to have some sort of magical properties to it, seeing as how El can see now, and he wasn't even the true owner. If it was in the hands of the true owner at any given point, I'd imagine it's properties would increase dramatically, to the point where, oh I don't know, you can make things out of nothing?
Sorry if you couldn't follow along at all, it's 1 am and I'm all over the place mentally and I'm also very tired.