Fiona was born to King Harold and Queen Lillian of Far,Far Away. When she was a little girl a witch put a spell on her that by day she was a princess and by night an ogre. The King and Queen were forced to lock Fiona in a tower guarded by dragon until her true love came and broke the curse. In Shrek Forever After the King and Queen were about to sign away their kingdom to Rumpletiltskin to break Fiona's curse until news came that Shrek and Donkey has rescued her.
Fiona married Shrek in the first film after he saved her. In Shrek the Third she said she was pregnant before Sherk left on his quest. At the end of the movie she shows she gave birth to ogre triplets two boys and one girl, Farkle and Fergus (Male) and Felicia (Girl) though their names were not said until the fourth film.
It in shown in Shrek, Shrek 4-D and Sherk Forever After that Fiona sings so beautifully that birds explode. Fiona also has amazing kung-fu skills that she inherited from her mother as shown in Shrek 3
In Shrek she is seen being rescued by Shrek and Donkey and goes on a quest to Duloc the during the climax she is seen as a ogre in the end of the film she sings I'm a Believer.
In Shrek 2 she is seen with Prince Charming trying to marry her and NOT Shrek but Fiona hit Charming.
Shrek the Third
Well you didn't think you got your fighting skills from your father did you
-Queen Lillian to Fiona
In Shrek The Third she announces she is pregnant .She is later seen rescuing Shrek from being killed in the play Happily Ever After After All play.
Shrek Forever After
In Shrek Forever After she is seen as a Hunting ogre with her hair (finally) down.