I love Kass, how would you think he’d play?
Have a couple of ideas but what i'm most sure is that i would give him the 4 champion powers that link uses in BotW in teh same way zelda uses the 3 link's magic in Ocarina of Time.
Up b would be revali's Gale, Side B would be Daruk's protection, Neutral B would be Urbosa's Fury and down B would be Mipha's Grace.
This could either be immediate more normal specials (Daruk's protection a hitbox that gives intangibility and reflects projectiles) or other way could be powerful speciasl with slow activation times. meaning that Kass would like to put some distance in order to execute those specials.
If that would be the case, Revali's Gale would be the quickest and work like a full charged Hero's UpB. Mipha Grace would be the slowest and cure 10% in a span of 5 seconds (getting hit cancels the effect), Daruk's protection would give kass a one hit Shield/Reflector kinda like bounce and Urbosa's Fury would be an explosion akin to a medium Magic Burst. Since this specials would have a big startup, Kass could sing a quick verse in order to activate them.
Since all of them can be very potent , Kass would need to get some breathing space in order to activate them and limiting the spam. Specially of Daruks protection and Mipha's Grace.
From there for his more neutral moves, you have 3 options
First is a combination of kicks and ussing his instrument as a weapon, with somewhat goofy but rather simple atacks.
Second is a made up attack style with wing sweeps (like falco), and maybe a Rito Bow for smash attacks and a couple of aerials.
And third, my personal fabvorite but would be the longest stretch, he using more of some kind of "channeler/Shaman" style where he quickly summon the champions spirits in order to attcack as he sings small verses. having slow but big disjoints, coupled with some quick tilts. (summoning daruk to do a big sword attack like Ike's as his side smash, summoning urbosa in order to do a shield bash akin to Palutena as his dash attack, summoning Revali in order to do a downwards bow attack similar to megaman's projectile as his down air, etc...)
He may or may not be constantly with his accordeon, but he doesn't uses he should have a long taunt similar to bayonetta's dances where he plays it.