Eighteen of the twenty DLC characters for Disgaea D2 appear in the Post-Game, where you'll have to face and defeat them. The exceptions are Fuka and Desco from Disgaea 4, who have stumbled into Laharl's Netherworld via one of those convenient dimensional warps.
Once you've activated the DLC, you can access Fuka and Desco by passing a new bill at the Dark Assembly called “Meet the Strongest Sisters,” which costs 300 Mana and has a very high chance of successfully passing. You may need to grind for a bit to afford it, but you can pass the bill as soon as the Dark Assembly reopens in Episode 1.
Doing so puts Fuka and Desco on your roster at level 1, with no equipment and at a three-heart starting Likability with each other. You can use them throughout the main game as you see fit.
Fuka is a half-crazed middle schooler with an affinity for Axes, and since she's convinced none of this is actually happening, she takes half damage from enemy special attacks. This includes spells, which takes a lot of the sting out of her low RES and -50% vulnerability to Water magic.
Since Desco is a monster, her high starting Likability with Fuka means her Death Co. Mounted Skill gets a 40% bonus to its damage. In the early game, Death Co. when augmented by Final Boss Setting is one of the most powerful nukes you have at your disposal.
This also lets you get a running start at the Public Display of Affection Trophy, which requires you to use a Mounted Skill with a monster and humanoid who are at maximum Likability with each other.
Check out the Raspberyl DLC Character Spotlight. The DLC code for Raspberyl is available in the Disgaea D2: A Brighter Darkness Official Game Guide.