While it would be nice to have another hero in that case, the beginning of the last Incident emphasized for me that just because we've recruited these heroes, it doesn't mean that they'll all get along. The constant adding of new team members probably didn't help, and adding someone new right when everyone is trying to recover from the previous Incident feels like something that might stress our heroes out, though probably not actually STRESS them out.
It might get better once we INTERACT more with our heroes and ask them how they're getting along with their teammates, but I suggest asking Subaru and Naoko about themselves and their families first. We haven't INTERACTed with them yet, and asking about them instead of their relationships with their teammates will hopefully show that we care about them more than just as cogs in the machine we need to keep running.