Here is a list of available Final Fantasy summons. If you want to use any of these, you must ask either me or Riku. All summons on this list only last for one post, activating their overdrive then leaving:
Anima: First appeared in Final Fantasy 10. It is one of the most powerful summons in the series. Overdrive: Oblivion
Bahamut: First appeared in FF10. It is one of the more stronger summons of the series. Overdrive: Mega-Flare
Belias: First appeared in FF Tactics, later appeared in FF12. It represents Ares, the Greek God of Fire. Overdrives: Painflare, Hellfire (depends on dice roll which one he uses)
Diablos: The dark Guadian Force from FF8. Overdrive: Dark Messenger
Ifrit: The fiery Aeon that appeared in several Final Fantasy games. Overdrive: Hellfire
Ixion: The thunderous Aeon that appeared in FF10. Overdrive: Thor's Hammer
Magus Sisters: The trio of bug-like humanoid Aeons that appeared in FF10. Overdrive: Delta Attack
Shiva: The frosty beauty that appeared in several Final Fantasy games. Overdrive: Diamond Dust
Valefor: The first summon received in FF10. Overdrives: Energy Blast, Energy Ray (depends on a dice roll which one she uses)
Yojimbo: The samurai Aeon that required money in exchange for his services in FF10. (The same applies here.)
Ultima: The final summon from the FF series, and also the strongest. If you want to use this summon, you must prove you can handle it in an RP. Overdrive: Eschaton