I really hope he is not the typical good guy that everyone loves and play the straight man to the other character's quirks like Robin did, but I don't have much fain in IS when it comes to this.
That developing basically 3 different FE games at once is too big of a workload and that IS will ending up cutting corners causing all of them to be subpar products. Also echoing Kirokan's post.
I wouldn't really give a shit. Like everyone else said, how the character is written is what matters. But, if they write them on the same level as Jill, I would be pretty elated.
I actually think this feature was included to try to encourage people to challenge themselves in the game since if they get completely stuck they could just lower the difficulty and not have to restart the entire game again. Well, that's my take on it.
My initial reaction to this was a huge "Whaa?", but after re-watching the trailer over again a lot and loving the song I'm actually really hyped for this. It's so weird that I'm really interested in it.
Interview on how the game got like it has:
http://www.siliconera.com/2015/04/02/shin-megami-tensei-x-fire-emblem-got-persona-like-style/
Looks like they're well aware this didn't capture the FE part well. (Though there was quite a but of subtle FE stuff in it.)
I'm glad this is something different and surprising and not some slapped together crossover trying despeately to please both fan bases like most cross overs tend to be.