Because Miku is an easily recognizable and marketable character that will attract non-persona players to this game.
Good ol marketing.

Actually, yeah, if Miku can get in Labrys deserves far more than a costume.

Miku's popularity waning, am I missing something here?

*Ahem* The issue of whether she should be in the game or not, I REALLY dig that design. She looks great in the Persona art style.

Anyway, newcomer to the franchise here so I thought I'd say hello. I'm very slowly playing through Persona 3 at the moment with my girlfriend since that she's already a fan, but a friend loaned me Arena today which I'm quite looking forward to playing.

Miku was probably included because Sega, the guys who kept Atlus alive after their last owners were caught paying them in imaginary money, might have asked if she could be in the game too. so a character who is famous for having her own rhythm games getting a cameo in another rhythm game is more than fine.

I too would like Sho, or Labrys, but Miku is perfectly fine, and the fact that she comes with her own remix is also a nice bonus.

I mean, if you check sites like AmiAmi and Hobby Search, her merch certainly doesn't sell like it used to.

My issue with Miku is that, if this wasn't a Persona game, which is a spinoff of SMT, I'd be ok with it if this was just some random rhythm game, but it's not, it's Persona, and I honestly think that fans of SMT proper are allowed to complain. I also know it likely wasn't Atlus' choice, but Sega likely went to them and was like "Hey, we make this rhythm game, put the character from it in yours" Like, as a marketing move, it's fine as it'll give Atlus money to make cool stuff like more SMT games, but as a fan of Persona and SMT, it sucks to see this anime girl I'm tired of seeing just thrown into a series where I feel she has no reason being there. Also, because I feel like she took away a spot from other Persona characters that could have gotten in.

As for the art, yeah, I like Shigenori Soejima's art, so the art does look nice, but still.

the thing is, Persona has pretty much become it's own thing, not an SMT spinoff.
in the same universe yes, but not necessarily a spinoff.

and there is no obligation for the other SMT characters in the game. they don't have to, they can choose who to add in the game. Miku is fine. calm down.

This came out yesterday.

https://www.youtube.com/watch?v=wvpOwQaqRXA

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAH!!!!!!!!!!!!!!!!!

(Also Mamoru Miyano)

Great trailer, I really like the general aesthetic of the UI and that pirate Persona looked awesome. The summer 2016 delay is unfortunate, but expected. Better that they take the time to really make it great though instead of trying to rush it out.