X - X 2
Final Fantasy 10 - 2 is a monumental landmark in the FF franchise not because it's great (though it is widely deemed as a good entry in the series) but because it was the first direct sequel to a FF game. FF10-2 revisited all the locations we knew and loved from FF10 and re-introduced us to Yuna and Riku, the former of which has changed considerably. Another thing that changed considerably was the gameplay, which was distinctly different to the strict turn based system in FF10. The job system was re-introduced, meaning that characters can change their role mid fight and have a totally different set of abilities. The battle system was like an early blueprint for the Paradigm system used in FF13 and FF13-2 however, you still had control over all three party members rather than just the leader. In terms of story, there is a struggle between two factions; New Yevon and The Youth League and there are some evil hijinks afoot as well. The real story going on here, or at least the one a FF10 fan will care about, is Yuna's search for Tidus or 'him' as you could change Tidus' default name in FF10 to anything you want, thus causing the writers to only refer to Tidus as 'him'. Well, you only get to be reunited with him if you play the game twice and you only get a reunion cutscene if you get 100%. Personally, I watched it on youtube.
Y - Yojimbo
Yojimbo is an optional summoning from FF10 but he paves the way for talking about the various references to characters from outside of the FF games. Yojimbo is the name of a very famous Akira Kurosawa movie about a Ronin playing two gangs against each other in a rural town in Japan. It was later bastardized by Sergio Leone's A Fistful of Dollars, which led to Kurosawa suing for copyright infringement and winning the international rights to the film. Kurosawa made more from those rights than from any of his own movies due to the films success in America. Now the history lesson is over, back to the summonings. Yojimbo isn't the only summoning that references something else, for a start Yojimbo's dog is called Daigoro who is the little boy in the Lonewolf and Cub series. The ice goddess Shiva is a reference to a Hindu Deity, the fiery beast Ifrit is a supernatural creature from Islamic culture and Odin...well we should all know who he is. Only the father of Thor. The writers of the FF games have had a lot of fun taking these diverse characters and myths and re-engineering them for the FF series. While they often reflect little or none of their inspiration's appearance, they are certainly fun and long-time fans of the series enjoy their return as much as the other conventions of the franchise. Yojimbo is an interesting summons for another reason, and that is you have to pay him to do anything. He is potentially the most powerful Aeon in FF10 as he has moves that can instantly cut in half even the final boss in one slash. Of course, you never know what he will do and he may just sick his dog them. Unlike the other Aeons, you have absolutely no control over his moves, aside from the option to pay more or less. It is kind of like a roulette game though, if Yuna is a high enough level even the weakest attack is pretty powerful.
Z - Zidane
Of all the FF games, FF9 is the most madcap and cartoony. There are loads of different species wandering around in seeming harmony, from hippo people to rat people. Normal people too, not to mention goblins and monkey people. Zidane is a monkey person, though to begin with he is the only one. Of all the FF protagonists Zidane is the most fun and adventurous, possibly a reaction to the incredibly depressing Squall. Zidane's weapon of choice is either twin knifes or a bladed staff crafted from twin knifes and he is also a thief. He also has incredibly powerful overdrive magic.