RedXIII was the thirteenth specimen Hojo expeiremented on and Hojo was expeirimenting on him because of his incredibly long life span. Also Aeris and Nanaki are the last of there races(supposedly). He was trying to breed Nanaki with Aeris(Not sure how) so he could study both the Ancients and and Nanakis race before either Aeris or RedXIII died.
And I agree that Nanaki(I called him Bongo) is a very cool character. I mean his tale is permanetly on fire!!!!!
How awsome.