Some random Ness-supporter: BUT THEY'RE TWO SEPARATE CHARACTERS!!
Well, yeah. But guess who else are two separate characters?
And this is the same exact thing in many other series, especially RPGs. It's a very recurring theme.
People will simply ask, however, "Since when was Lucas and Ness the same concept?" Well, someone hasn't played a Mother game before and if so they missed one of the major factors of it, especially the main characters.
Much like Zelda, each main Mother character is literally the same concept. In Zelda, we have a silent protagonist named Link, who ends up wearing a green tunic and wields a sword to go on some adventure to save the world from Ganon/Vaati/whatever. Sometimes even having to save Princess Zelda or something. In the Mother series, it always stars a young boy kids can relate to with a stripped shirt, similar facial features, same exact stats in pretty much every way, same exact basic moves, equips the exact same items, etc. Pretty much, every is the same with the Mother protagonist.
(Yes, the Lucas one is fanmade, deal with it.)
As you can see from each one, they're pretty much the same overall. The only difference is, with Lucas, he looks drastically different, and this sets people off quite greatly, really. Do NOT be set off by his looks alone, he is indeed the same exact concept like every Link. Yes, he's a different character, just as much as Ninten is a different character. Yes, Link in OoT IS a different character from Link in TP. Now, I dunno about you, but this would pretty much make me just rather have Lucas than having both him and Ness. That'll be the same exact thing as having both Ninten and Ness in at the same time, and I'm sure no one in hell wants that.
Yes, if Lucas happens to replace Ness and take all of his moves, he really wouldn't be much of a newcomer, really. I can honestly completely agree with that. But, logically, yeah, he does count as one, because he does have a different name. Even with each Mother protagonist having different names, they're are STILL the same concept. Anyone who's played each game would simply know that. In terms of actual fighting ability, personality, ideal, they are, indeed, the same character.