Aside from leveling up, you can collect materials to gain passive skills for your characters. (gametactics.com)
Choose from over 1,300 passive skills spread out on the iconic skill tree and even socket jewels to modify the tree itself. (player2.net.au)
In the case of Dungeon Souls, you earn passive skills as a reward for continually playing with a character, so it behooves you to learn the intricacies of each character class. (blog.dropleaf.io)