Apologies for the incoming wall of text.
If the SMT rep gets demon summons, they would most certainly have Jack Frost and Pixie among them, though since they're so iconic maybe they could be integrated into their other specials. For example, the side-special could be Jack Frost with bufu as an area of denial move (it would be cool as a Din's Fire/PK-Freeze-like move, just more horizontal) and the up-special could be Pixie appearing and using zan to boost the player up (I'm imagining it working like Robin/Hero).
As for the down-special, I'd personally love to see a loadout of demons appear (like hero's spells). Selecting them makes them appear and use a single attack before fading out. It could be interesting if there's a tiny delay between the demon appearing and it using it's attack so the player can move and set up combos involving the demon's attack.
Of course there are wayyy too many iconic demons to feasibly put them all in, so if technical limitations only lets up to four demons be in the summoning pool, I'd choose:
Belial with Maragidyne, Thor with Ziodyne, Beelzebub with Death Flies (w/o the insta-kill, maybe gives a poison effect), and Metatron with Holy Wrath (chance to inflict extra damage on hit)
Of course those are just my personal choices, other demons that I could see being added instead are:
Decarabia, Ose, Samael, Girimehkala, Isis, Matador, Trumpeter, Mother Harlot, Mot, Archangel, Odin, Throne, Loki, Alice, Mothman, Mara, Mastema, and Yo****sune to name a few off the top of my head.
Note that I'm imagining this more for a Flynn/SMTV protag character, as Demifiend would probably play very different.
