I think even just making another sissel buddy with a different attack is more than enough really.
IDK about the details of making assists, but I am sure that making them do more than one thing is already difficult

If it's possible, sure, I can see that. Perhaps this assist could cycle through other attacks from the game? It seems like a pretty fun idea.
I wonder if there'd be enough demand for a patch like that though? If there is enough interest I will consider it, since I think there is merit in the simplicity of simple one-move assists. :)

I would give for multiple sissel assists based on some of his actions in the game
Maybe even combining it with an extra missile assist for swapsies

Sissel as a fighter is... complicated. In Ghost Trick, all of Sissel's actions are performed in his spirit form, where he manipulates objects by possessing them. He isn't exactly a directly confrontational, hand-to-hand combat guy. In fact, he literally can't fight physically, since, y'know, ghost.
So, I opted for an assist instead. Not necessarily because I thought it would be easier, but because it seemed more faithful to his role in Ghost Trick.

@Shipper65 It's probably just because the author felt like it. I'm personally not really fond of the assist buddies but they do seem like an easier way to put characters in, mainly because you'd only be making one move and not an entire moveset.

Why is he just an assist

YOOOOO MY HEART JUMPED!! THIS IS AWESOME! (i normally never beg ever but please make him a full character plz fellow ghost trick fan)