In game there are quite a few ways that you can collect more Fatima, and if you want to learn all about them, then let me give you a hand, because that will definitely come in handy! You can get more characters in Dark Sword 2 by completing the following:

Every time after finishing single missions, you will receive a few rewards, among which you can have Cores too. The rewards are random, so it's not like you will collect Cores every time you complete Single missions. You also need to make sure that you have enough slots for all the loot! If you don't have slots for all the loot, you can sell some stuff or choose not to pick it and instead pick the Cores.

From certain stages from the Squad Missions you will receive some certain Fatima Cores. The Squad Missions are more difficult than the single ones, and they can only be played with the whole squad at once (hence the name).

You can get Cores by opening Supply Cubes. It's random, so you can get either equipment, gears, stuff like that or Cores for Fatima. I suggest saving 300 Souls so that you can buy 12 Supply Cubes, instead of spending 30 Souls to get them one by one.

If you are a Premium player, you can buy the Cores directly from the Shop, with real life money.