Business cards?

[cj iwakura]

What are they for? I assume it's involved with the Compendium how, but is there another use?

[Pibbman]

Even at the point I am in the game right now, I've still yet to see anything to do with them. There are also other things that have been given that I haven't seen what their use are for. Like the twigs, which I'm sure you've found a few by now, or the other items that don't show up under your items area in the menu.

[Elizabeth]

I thought I read somewhere that they reduce the cost you have to pay to summon them off the Devil Chart.

[Pibbman]

That could be it, I've never tested it though. It definitely sounds like that is probably what it's for.

[Onion of Mystery]

I wasn't involved with the localization personally, but while I was watching over the testers' shoulders I noticed that demons would notice that you have other demons' business cards, which could affect your negotiations with them.

"Oh, you know this guy? Cool, I bet we'd get along."
or
"Wait, you've worked with this guy? I hate him! Get out of here!"

[Zambatoh]

Even at the point I am in the game right now, I've still yet to see anything to do with them. There are also other things that have been given that I haven't seen what their use are for. Like the twigs, which I'm sure you've found a few by now, or the other items that don't show up under your items area in the menu.

For some reason my Tam Lin used Demonstrate to turn those twigs into Wooden Blocks, which also helped me close a case. :)

[Elizabeth]

...They didn't really bring them up at first (demons tend to ask about my title), but now that they're interested in my Business Cards negotiations are not going well for me. I tell some demons that they were given to me and they get ticked since they claim to be stronger, and with other choices they're pretty displeased as well (curse randomness!).

[Wiseman4545]

I can tell you that they don't drop summoning costs, but I have heard something about them being used in some sort of demon card mini-game. I have not come across it myself so don't know how true it is.

[ShinMog]

Funny, I was thinking about asking this same question but I'm glad I'm not the only one. I even checked the instruction manual (which was very funny as others mentioned), but the business cards are only mentioned in passing there.

I wasn't involved with the localization personally, but while I was watching over the testers' shoulders I noticed that demons would notice that you have other demons' business cards, which could affect your negotiations with them.

"Oh, you know this guy? Cool, I bet we'd get along."
or
"Wait, you've worked with this guy? I hate him! Get out of here!"
I've noticed this happening a few times in my game also, but I really was hoping that it would be something more than just this, if not now, but at least later in the game.