People Person Achievement
Play with 10 different co-op partners for at least 15 minutes each.
NOTE: This achievement can only be obtained with players with the NTSC-J version of the game, whether it be online or system link.
This does not have to be done on the same character or during the same playthrough. You will generally find new games popping up all the time, so start one yourself and then just latch onto someone for fifteen minutes. Once your achievement tracker pops up, repeat the process with nine more people. To make it go a little faster, try to find open games with 2-3 people at a time.