Ariana Griande is Grian, not a persona like the hermits think. Because Grian is a parrot hybrid, he has the ability to mimic any voice he hears with practice of course.

One day, while walking through the hypixel lobby, he heard a girl singing and though it sounded nice and subconsciously practiced her voice.

Now when ever he "switches" over to Ariana Griande, he uses the voice to make it match the outfit.

Only scar knows about this because he asked him about it, the other hermits just live with it