Yojimbo is the first of three optional Aeons that you can obtain. He is the easiest of the three to obtain and can be picked up during the regular story mode when you pass by the Cavern of the Stolen Fayth. Check out the Secret / Optional Aeons section in the side quests area for more information on how to obtain all three.
Yojimbo is not like any of the other Aeons in the game. You have to pay Yojimbo each time you want him to attack and his attack will differ depending on how much or how little you pay him as well as if your alignment with Yojimbo is high (not letting him die, never paying him 0 gil, not immediately dismissing him after summoning him). Your alignment with him will rise the more you use him. He has the following attacks:
Daigoro: Yojimbo’s weakest attack - his dog will conduct the attack for him. This is Yojimbo’s indication that the amount of gil you provided to him was not enough.
Kozuka: Yojimbo throws throwing stars at a single target.
Wakizashi: Yojimbo’s weakest attack - his dog will conduct the attack for him. This is Yojimbo’s indication that the amount of gil you provided to him was not enough.
Zanmato: Slices enemies in half ending the battle instantly (similar to Odin / Gilgamesh from Final Fantasy VII)
Yojimbo will be able to exceed 9,999 damage after Auron has obtained his Celestial Weapon, Masamune.