I haven't really used Nitori much. I like how her abilities work, and I have her grinded up to a decent level, but Its hard me to use her in the long run. In a way this kinda reminds me for suikoden games, a lot of characters, but you only use a select few along with storyline requirements. only thing is, Suikoden has 6 characters in the battle-party. So far my main party is almost always Momiji, Youmu, Patchouli, and the storyline character (if no storyline character, someone I'm grinding atm). Not sure how hard its going to be for me to pull myself away from that team, unless I need to replace Patchouli/Momiji with a more Physical/Magical based character respectively.
Another thing I noticed so far, that quite a few bosses that I fought don't have really noteworthy steals. there's a couple, but its mostly just use-able (restorative) items. This is the main reason why Marissa isn't a main party member for me. I don't know if the steals get better or are worth it later in the game, but that's something I kinda don't like. The best counter I can think of that is, if you don't want to give them equipment to steal, (something you can possibly buy in the next area or area after) or rarer skill scrolls, you can always to to give permanent stat boosting items respective to their type. Only if it doesn't make you to overpowered (which i see hard anyways). I mean there's so many character that quite a few people might play differently so those items can be distributed a little better. Like i said though, I don't know if this gets better later in the game because I really haven't gotten to far into it, but its turned me off from trying to steal from every boss like I normally do in RPGs.
Of course it's up to you on what you do, I'll enjoy the game either way, it'll make things less tedious to try to steal from everything, or bringing Aya into the party to check what they have to steal, then reload game to set-up actual party (unless you are an Aya fan then you'd be fine). Well this was a long post from me, I need to go to bed now. Once again keep up the good work, and I'll keep playing it as much as I have time too.