I actually never tought about it that much but i do have a few ideas on how this could work. I think he would be a good addition if done correctly of course, like perhaps he could joke with the fact that they used to call him "zero" and get a chameleon type of suit that blends in the background, use stealth and boost based attacks (since the abilities you get by going forward with his confidant are boost based) or do the opposite contrary by getting a suit that stands out (i dunno a bright yellow pop singer suit to fit the theme of being in the center of the attention) and use a persona with light or electric based attacks. As for his codename depending on which version he could be either Umbra, Shade, Admin, Mastermind or Spotlight. This is just something out of my imagination tho lolol.

I heard that Mishima was suppose to be a phantom theif (also Togo, but I believe she was replaced with Makoto) but got scratch due to time constraints. In terms of suit a magician seems the most interesting and fits him with being the center of attention, with this he can help you make more gadgets. Codename could be magician or performer etc. Or something.

I think Mishima would be great with magician suit but with bright colour since it will reflect back his hidden feelings, as for his name it could be something related to infuence cause thats how he boosts the team and his Persona could be Spartacus (from ' zero' to ' hero')