If I may say so... The only character I see being related to Mother in the closest possible way is Lucas...
Despite of how much you hate him for some reason, I think he would be a good choice of partner for Ness. I say this because I want to stay true to the Mother series like I have since the 64 Smash Bros. and I will continue honor that by playing as both of them as my mains.
Come on, is Lucas is a well deserving partner for Ness! Hell, the sword-only nubs are clamoring over Marth and Ike teaming up (OMG Blue Hair and Swords!!! *screams like a fangirl* *faints*)... ugh... Zero originality...
I can't think of a characters more original than Ness and Lucas, Kids with Psycho-Kinetic Powers who use Yo-yos, Slingshots, and a wide range of assorted Blunt Objects clubbing Aliens, Obscurely dressed Pig-like Soldiers, and most importantly... Hippies (nothing is more gratifying than owning a hippie with a bat to the face, SMAAAAASH!!)
These two need to stick together, this squabbling between the Ness and Lucas players is pretty pointless...