Sephiroth's last name was never mentioned and is not included in the game manual. It is unknown if he even has a last name. However, his parents are Professor Hojo and Lucrecia Crescent, so either "Hojo" or "Crescent" could be his last name.
Rikku does not have a last name.
Reno's last name is Sinclair
They never tell you his last name. The characters in Final Fantasy X are only given their first names.
Yuffie's last name is Kisaragi.
Barret's last name is Wallace.
Cid's last name is Highwind.
AeriS' last name is Gainsborough.
Vincent's last name is Valentine.
Tifa's last name is Lockhart.
There is no place in Final Fantasy X or X-2 that says he has a last name. In fact, no one seems too have one.
When the first one came out, it was the developer's expected last game, hence his 'final' fantasy. It turned out the game was a hit, and more were made, but they kept the name.
Strife is the last name of Cloud in Final Fantasy VII. The game was released for the PlayStation in 1997.