I'm not sure that "unique" or "interesting" are even the right words to be using. In some ways, they still matter, but maybe "individual" is more at the core of it? Lucas is a pretty weird addition. Same abilities as Ness, looks almost identical. He's rather obscure too, compared to the rest. Now, his moveset isn't what propelled him to a spot on the roster. I've wondered that maybe it was Sakurai's attempt to get Mother 3 more recognition for an international release. Maybe it was just going back to his idea of adding Lucas in Melee. BUT WHAT REALLY SETS HIM APART FROM NESS...
It's his personality, isn't it? Lucas is more unsure of himself. He has that... meekness about him. It's brought up a lot too. His trophy and Snake codec mention how he "grew stronger." It's something Sakurai was trying to push forth. Aside from his moveset being much different than Ness, maybe even something that... subtle? is what Sakurai also looks for when trying to make sure every addition feels like an "individual."