If your familiar with FFVII and its characters back on the old PS1, I highly recommend
Crisis Core: Final Fantasy VII
Its a must have game, one of the best RPGs and definitely my favorite psp game, it will remind you in the good old rpgs like chrono cross and ff7 but this time with epic graphics..
I was highly addicted when I got it back in 2008 :P
Breath of Fire III PSP
If you never played Breath of Fire III on the PS1
Kingdom Hearts: Birth By Sleep
This one isn't totally an RPG but its a must have Square Enix title
Great graphics and takes 40h+ to beat, like usual RPG games
Im sure I missed alot of great RPGs on the psp, but thats what I really remember right now..