Tbh *SPOILERS* Kinda wish Tsukuyomi was the partner till the end somehow, its not that I dont like Aogami but it would differentiate both rutes more since both the godesses and partner are out for revenge
im imagining how awesome it would have been if we got a game that had nahobino rules, but you're also an ordinary devil summoner that gets to become a different nahobino depending on which route you chose. sounds pretty ambitious and I wouldn't be surprised if that was probably one of the ideas they cut from the game.
This is actually the battle theme for when you're in the new form. If you're familiar with OG V's OST then this theme basically tells you what the deal with the new fusion is.