He doesn't have one, No one in Kingdom Hearts does.
Axel, or Lea, as he was known prior to becoming a Nobody, does not have a last name in the series. It has never been revealed or shown in any installment of Kingdom Hearts.
Lea
Sora's last name is never mentioned in the Kingdom Hearts series.
Axel's name was Lea before becoming a nobody. This is confirmed when Saix refers to him as Lea during a cutscene in Kingdom Hearts 358/2 Days. More information on Lea will be available with the release of Kingdom Hearts Birth By Sleep.
Sora does not have a last name, he is 'just' Sora.
Xemnas
Doubt it. Kingdom Hearts: Birth By Sleep (same as KH3) takes place nine years before the beginning of KH1. Axel probably was not even existant then, although his Somebody, Lea, probably was, but Lea has no resemblance to the game except that he is Axel's true self. So the answer is NO. Probably. I am 99% sure he is NOT in it, but don't get mad at me if he is, which is quite unlikely. Actaly(SPOILER) Yes he will but not as axel his name will be lea
The scene that we see in Kingdom Hearts II Final Mix+ and the Opening in Kingdom Hearts 358/2 Days, where Xemnas gives Roxas a name, takes place right before Xemnas brings him into the Organization and introduces him to Axel. It's a flashback, so to speak.
that was his name when he was growing up remember it happen a while ago so they where still kids Axel is leas nobody if u don't no what that is look it up sorry hope that helps
I know one is Pictures of You by The Last Good Night
Riku's last name was never mentioned in the Kingdom Hearts series.
Kingdom hearts birth by sleep, kingdom hearts, kingdom hearts chain of memories, kingdom hearts re coded, kingdom hearts 365/2 days, kingdom hearts 2 and kingdom hearts dream drop distance. those are all the game s there is kingdom hearts 2 final mix and kingdom hearts re: chain of memories