I love this question! Assuming that all of them /would/ try and play it and were physically able to:
Avery is a Timmy-- they're playing gief and all they wanna do is SPD.
Arya would play around with the cast and be drawn to how straightforward and effective Marisa is, but would 100% stick with Ryu once he learned about his character. (The blue fireballs help too)
Fenn would normally play JP, but against Arya they'd pick Honda and spam headbutt to try and tilt him.
June would play Dee Jay simply because he looks cool. He would flail wildly with during his rhythm super in the rare event he ever does the input to activate it correctly.
Harley would play Luke because he's on the cover and doesn't wanna put in any more effort than that, they're just playing to spend time with others.
Jazz would play Manon because they think her throw animations are pretty.