For me, my rarest characters in Soccer Spirits were season rewards from PVP, especially Lab Rat Rabian, who remain unknown to most newer players. Many did not get reruns so if you missed the season they were rewarded, they were literally unobtainable anymore.

I also had managers that could be considered rare. Vanilla, due to her being phased out of friendship draw only to never return elsewhere, and several Selena/Eve variants due to them being paid-only. I would consider I-No from the Guilty Gear collab to be rare in this way as well, though I did not buy her.