It seems to me that Laharl was NIS's personal favorite back in the early days, when Disgaea's popularity took off and landed them a substantial market outside of Japan. Nowadays NIS is starting to put loads and loads of recurring characters, so the argument that Laharl gets to show up when someone else's favorite character doesn't is a little moot (even Ash finally showed up!), unless your favorite character is one that hasn't shown up ever since their debut at all, but I can see where people are coming from.