The second game has new characters, but the characters from the first game do show up later on. You don't NEED to know who they are, per se, but it makes things a lot more interesting.

Also, Disgaea 1 has the superior story but Disgaea 2 has the superior system, so I imagine it'd be really hard to go back to Disgaea 1 and play without all of the tweaks they did to it so that it works better.