@pooya_d: Each Final Fantasy is separate from the previous one. There are a few exceptions but they are obvious as to which ones those are (X and X-2, XIII and XIII-2 and Lightning Returns, etc...). XV is a completely new cast, story, combat system, et al. You won't miss out on any of the backstory or anything by not playing any others. If you like JRPGs, I do HIGHLY recommend many of the previous ones. FF VI, VII, and IX are considered the best Final Fantasies, but everyone has their own opinion. XV is new to all of us, but might give some of us nostalgia as they like to incorporate similar things in each FF (summons, moogles, Cid).