From what i know it's totally random.
I got Arcana from the troll guy while i was doing the "Save the princess" quest.

I got no girls from the ninja and fighting him from more then 1 week, like 3-5 times per day with my 6 energies = 9-15 battles.

I'm probably unlucky and since i ended the quests i'm only logging in to battle them and pachinko.

So as all already said is pretty random couse i got 1 girl out of 6 in a mounth.