How big of a case do you need? What are you planning on carrying in it? And how much do you want to spend?
Since you want a passenger, I assume more room is good, so you should probably get the Amotostuff mounting plate, because it's adjustable and allows for a couple extra inches of passenger room.
As far as what case to go with it, well, there's an insane amount of options, but if you use that plate you need to get a Givi Monokey case. It just depends on what size and options you want, and I believe all Monokey cases have the backrest as an option.
Personally, I'll be getting a Shad topcase because they're significantly less expensive, and appear to be of good quality. I rarely ride with a passenger, and when I do, she's clinging to me for dear life, so the extra room afforded by the Amotostuff plate isn't necessary.
A lot of people report that a backrest pad isn't necessary if your passenger is wearing a jacket with armor, just something to think about. That would give your passenger the most amount of room possible.
Here's the case I'll be getting:
$175 shipped from adventure moto, and includes mounting plate. The backrest is also an option, for $35.99 but I'm going to try it first without the pad.