Hell (HL) is the currency of the Netherworld. You gain HL from every enemy you successfully defeat, just as you gain EXP and Mana; the HL from defeated enemies is the sum shown in the results screen after you’ve cleared a stage. You can also earn increasingly high sums of cash as rewards from the Bonus meter, by opening treasure chests, and by selling unneeded gear to merchants and other vendors.
HL is used to buy equipment and consumable items from the Rosen Queen Trading Company, which has a branch office open in Laharl’s Castle near the mess hall; to bribe senators in the Dark Assembly in the event a bill fails to pass; to buy rare items and Innocents from well-hidden shopkeepers in the Item World; and to put forth proposals in the Item World’s equivalent to the Dark Assembly.
In general, money in D2 is a feast-or-famine deal. Either you have next to none or you’re drowning in the stuff. The fastest way to make cash in the early game is probably to run Concert Stage in Episode 5 or later, as it’s populated by easy-to-kill monsters that are thoughtfully standing on Geo Panels that cause them to drop more HL than usual. Selling obsolete or unused weapons and equipment is another decent way to raise funds, particularly after runs through the Item World. You often come out of random dungeons with 10 to 20 floors’ worth of stuff.
By the time you’re regularly exploring the Item Worlds found inside high-end weapons and armor, though, money is easy to come by. A single enemy unit’s HL value is determined by its level, so the higher an enemy’s level, the more money it’s worth when it dies.