Yojimbo is an optional Aeon and can be found in the Cavern of the Stolen Fayth, which is accessible after defeating Defender X in the Calm Lands. Once you enter the cavern, you will have to make your way to the back, where you will be forced to fight Yojimbo. Upon winning, the teleporters will begin working again and you will be able to enter the Chamber of the Fayth. Unlike other Aeons that will join you upon meeting them, Yojimbo will require a little bit more effort.
In order to actually recruit Yojimbo, you will have to fork over some money. Initially, Yojimbo will ask you a question and if you pick the third option, then he will tell you an amount. Cut this in half and add a single gil to make an offer, which should get Yojimbo to reduce the amount needed. From here on out, things seem to get a little trickier, as sometimes he can flat out refuse your offer, kicking you out of the room. Of course, you can go right back in and start out the offer thing again. The best thing to do is cut the first amount in half, as well as the second, then do 70% or so of the third. So, you would go 125,001 for the first, 112,501 for the second, and 141,752 for the third. This should net you a price of around 190,000 gil.
Whenever you summon Yojimbo in battle, you will notice he is different to the other Aeons. The only two commands for Yojimbo are Pay and Dismiss. Yes, you will have to pay him gil in order for him to attack. If you happen to pay him 0 gil, then that will also cause Yojimbo to leave. You cannot directly control Yojimbo and he cannot use Shield or Boost, so he is quite vulnerable to attacks because of this. Whatever you pay him, he has the opportunity to use one of four attacks, which are as follows:
- Daigoro - Yojimbo will send his dog to attack the enemy. This is his weakest attack and it will not work on long range targets.
- Kozuka - Yojimbo pulls out some special throwing knives and throws them at a single enemy.
- Wakizashi - Yojimbo will quickly unsheathe his sword and slice at either a single enemy or multiple ones.
- Zanmato - Yojimbo will instantly kill all enemies in the battle. This will work on anything in the game, although bosses with multiple forms will just continue into their next form, unless they are in their final form. You can tell he will use this, as he nods upon Yuna giving him the money.
So, without going to the nitty gritty details, there is a lot of math and factors involved in Zanmato’s attacks. One thing that is factored is the amount of gil given, as well as a hidden relationship parameter to Yuna. The latter is definitely important, so make sure that Yojimbo doesn’t get KO’ed by enemies during battle. Also, enemies in the game have a hidden Zanmato resistance, so that can be a problem, too, especially against foes like Penance or Nemesis. If his Overdrive gauge is full, that does help some, plus you will also have to deal with a random factor.
You can see the good relationship he has with Yuna by the attacks he chooses, as Daigoro means the relationship isn’t that good and something like Wakizashi often means it’s in good standing. Note that there is also a chance that Yojimbo will attack for free, without having to pay him. If you wish for him to exceed 9,999 damage, then you will need to obtain the [Masamune] and at least use the [Mars Crest] to power it up some. You cannot add any abilities to Yojimbo using items.