Tix is one of your stats. Unlike other stats, Tix is not increased by Shop Upgrades. They are only increased by killing enemies, like minions, golems, and other characters. Tix is used to buy upgrades to other stats. Everyone gets 450 Tix when they join the game. Killing minions gives you about a moderate amount of Tix, golems give you more, and players give you about 100.