I think Demi-Fiend should be the fighter due to his popularity and recent remake on the way. He looks like he'd be the coolest-looking, and I hope he has some summoning techniques where he calls forth a set of demons for specific attacks, maybe like assists in Capcom VS. games. You press a special button corresponding to the demon, and they appear and act independently while you're still able to move, and there's a cooldown period before you can summon that assist again. That's what I'd want the Summoner to do too, if they were chosen instead.
For the demons that accompany Summoner/Demi-Fiend, I'm thinking Jack Frost, Pixie, and Cu Chulainn would strike a nice balance of popularity and practicality. Neutral special is Jack Frost. He appears in front of the player and casts Bufula, which is an ice spell where he blows an icy wind that extends at mid-range and does slight ice damage with a chance to freeze, sort of like Ice Climbers' down special. When performed in mid-air, Jack Frost will be suspended in mid-air while casting Bufula and will be flapping his arms to keep himself afloat. He has a fast cooldown. Down special is Pixie. Her main utility is to heal, so her skill will be casting Dia, a light healing spell that heals somewhere between 5% to 10% of the player's health. She has a slow cooldown. Side special is Cu Chulainn. He is best known for his physical combat, so his skill will be Brave Blade where he lunges forward from the player's position and strikes the earth with his spear with devastating force. Imagine the grounded animation of Corrin's side special, but Cu Chulainn is much taller and lunges a greater distance. It has a decent splash zone surrounding the collision between the spear and the ground that does decent damage, but if the spear strikes the opponent directly, it deals massive damage and has a high launch rate, making it a viable KO move. When summoned mid-air, Cu Chulainn will perform the same action as he does on the ground, but a direct hit from the spear will result in a meteor smash. He has a moderate cooldown. Up Special can incorporate a 4th demon for the recovery. I think it should be Mothman because that would be funny. The player would grab the legs of Mothman as he flies them back to the stage.
Whether or not we get a playable fighter, there should be a Jack Frost Mii costume regardless. He's the mascot of the whole SMT brand, and his proportions would fit perfectly for the premium model, specifically Mii Brawler.
I like Persona as much as the next guy and was super excited for Joker, but for the parent series to not be recognized in any capacity is honestly detrimental to Persona since the demons are just as important to Persona's identity as anything else.