The more I think about this, the more genius it is.
Brave Exvius is all about unlocking characters to use in battles. To unlock characters you have to use in-game currency, which are earned at an fairly slow rate by playing the game, or just simply bought with real money. Each unlock is a complete lottery as to which character you'll get - basically it's like you're opening Kinder Egg toy capsules and hoping to get one of the collectible figurines and not a trout jigsaw (incidentally, capsule toys are another big market in Asia... see where this is going?)
Ariana has a huge following in Japan. Mobile games are extremely profitable in Japan.
In the west shes a huge pop sensation and the mobile market is constantly growing. Social RPGs still don't do all that well in the west, with Kingdom Hearts UCX, FF Record Keeper and Brave Frontier being the outlier successes. Square owns the first 2, and incidentally the 3rd - Brave Frontier - is made by the same company who Square partnered with for Brave Exvius - might as well go for the hat-trick and corner that market space right?
Get Ariana to "organically" market her involvement via her own social and watch the bankroll tick in.
Even as a fan of the series, it's probably very likely that Square Enix has to pay some sort of royalty to her/her agent/her record label for the use of her likeness.
So the likelihood is she will either be:a) Free, either as a usable character or a plot driver. In the hopes that her legions of fans will download the app, give it a go, and it's free-to-start structure will entice enough of those users to fork over some money to progress.
b) Part of the random lottery, maybe even a "premium bundle" that has a higher chance to award rarer characters, to entice people into spending small, but frequent, amounts of money in the hopes of unlocking her.
In return, her inclusion in the game might encourage people to check out her stuff, maybe net a few sales, and more than likely some sort of contractual profit sharing based on the number of sales during her availability.
There's nothing altruistic or empowering about it. It's a smart business move. It'll piss off franchise loyalists, but to be fair a lot of franchise loyalists already think of the mobile titles in great disdain anyway, and they are probably in the minority when it comes to the revenue generation of these products.
As someone who likes her games firmly separated from reality, and as a result finds celebrity endorsing/shoehorning to be sleazy at the best of times, I struggle to get behind this - but I can't deny it's a smart business move. In the long-run it isn't really going to affect me as mobile titles (in general) hold very little interest to me. So, cool? I guess.