Beatrix, Blank, Marcus and Cinna can be permanent party members if you use the appropriate codes.

http://db.gamefaqs.com/console/psx/file/final_fantasy_ix_beatrix_scc.txt
http://db.gamefaqs.com/console/psx/file/final_fantasy_ix_bmc.txt

I agree with OD, Fratley should've been more involved in the game. Other than him, the only character I can think of at the moment is General Leo (FFVI).