The Counterpart Heroes DLC is indeed only available for Dungeon Defenders on PC.
Trendy Entertainment, the developer, has explained that small DLC packages like this are both too time consuming and too expensive to release for consoles due to the costs imposed on the developer by Microsoft and Sony.
"Those systems as great as they are, they’re still closed. You have to jump through a lot of hoops, even for important stuff like patching and supporting your game. Those are things we really want to do, but we can’t do it on these systems. I mean, it costs $40,000 to put up a patch – we can’t afford that! Open systems like Steam, that allow us to set our own prices, that’s where it’s at...."
After this price was posted on the Trendy Entertainment forums, Trendy's Jeremy Stieglitz replied:
Hehe yep, though it "only" costs us $10,000 to patch a Downloadable Game, at least :P
This is in addition to the strict requirements for patches and DLC imposed by Microsoft and Sony. This has been discussed on the Trendy forums and even described as a "train wreck" by Gabe Newell of Valve.