This is the 5117th most frequent Turkish word.
hikâyesi
The Turkish word "hikâyesi" means "his/her/its story" or "story" in English.
Here, 'hikâyesi' indicates possession and refers to 'Ali's story'.
Ali'nin yazdığı hikâyesi çok etkileyiciydi.
The story written by Ali was very impressive.
In this sentence, 'hikâyesi' refers to the story that belongs to each character, indicating individual ownership.
Bu kitapta her karakterin hikâyesi anlatılmıştır.
In this book, the story of each character is told.
'Hikâyesi' here is possessive, referring back to the third person singular pronoun (her/his).