Rei says something about diffrent stars assigned to the followers of nanto roku sei ken = south star(havnt seen all episodes). He was given the justice star. Rei is one of them and then the show shows us another two persons. Now, we also see the shadows of three other people in the nanto roku sei ken.
Who are they? Did the series show us that? Are they more powerful than Kenshiro?
The six Nanto Seiken masters (or Nanto Seiken's Holy Fists) are Shin, Rei, Shu (the blind guy), Uda (the pink treacherous one), Sauzer and ... Well for the last one, I don't want to spoil you, so try to read the mangas, they're great
And no they're less powerful than Kenshiro, even Sauzer who has a secret that allows him to resist Hokuto Shinken (that's because of this secret that Raoh doesn't want to fight with him)
Omae wa mo shinderu
there's one more Nanto master.You will hopefully figure it out in the manga or the TV serie.Its called "The neglet Star of Nanto"
and there are about five more,but not spoiling that