Hatsune Miku

A very well known music artist in HHgregg as well as the only employee at Wendy's other than Katsuya

Specifically, the Miku that shows up in Persona 4: Dancing All Night.

Back in her original dimemsion, she was a very well known song writer. Whether or not she was actually REAL in that dimension or a shadow in the TV world (which I've seen a few theories/headcanons abt) is something I'm not sure about, but she existed in some way there! One night post-concert, when Miku was settling down, she was teleported.

What she doesn't know is that she was teleported directly by Nyarlathotep. Not by random chance or by one of his arbitrary rules.

In any case, after being teleported she became extremely disoriented in the new world and distressed with the whole thing. Being brought from a world where you were well-known and liked to a world where no one even knows you is. Harsh. So, she was admitted into therapy! Cookie Man's therapy. The one that was actually good.

Through that she got help settling into the new world and additionally started gaining a new audience + traction through Cookie Man's encouragement. She got a job at Wendy's to make money while she worked on getting back something at least resembling her old life. She eventually stopped going to therapy and started living a new life here :-). Something about the whole "not being able to die" system broke something in her and she's grown easily violent and tends to try to keep everything calm by smothering the problem. With a shotgun.

Since then she's stayed working at Wendy's while balancing her music career, but while she's grown content with her life here, a part of her still wonders about her old, more simple life and a longing to return to it.

(Neutral-Negative) Sephiroth - She’s flattered by Sephiroth’s admiration for her, but she is also reasonably concerned about the therapy he’s running, seeing how his clients act and talk about him. The main reason she substitutes for him is just out of job opportunity and to get rid of him at least for a little bit, for the Therapy Gang's sake.

(Neutral-Negative) Goro Akechi - She isn’t as focused on rivalry and competition as Akechi, and she just sees the Wendy’s business as another business. However, she is frustrated at Akechi for constantly defiling the restaurant (e.g. nuking it and trying to claim the plot as his own) as well as the hints that he treats his employees badly. Definitely negative, but it’s not blind-rage like Akechi has for her.

(Positive) Cookie Man - Despite how she acted while substituting for Sephiroth's absence in therapy (she was just following Sephy's instructions), she's actually decently close with CM. Sees him as a mentor figure since he helped her set up her new life in HHgregg (restarting her musical career, encouraging her to seek out the Wendy's job, learning hhgreggan, teleporting, etc. etc.) and also generally told her about a lot of stuff that most other entities don't know much of. They still hang out a lot, every now and then getting lunch together or something.