How to Obtain Yojimbo in Final Fantasy Xby Nate Chang, eHow Expert ; Updated September 22, 2017
There are three Aeons (five if you want to get technical) that are not on your normal route in Final Fantasy X: Yojimbo, Anima and the Magus Sisters. All of them have unique and extremely powerful abilities that will greatly aid you along your pilgrimage. The only trick is getting to them. We'll start with Yojimbo.
Head to the Calm Lands, then through the northeastern exit, as though you were moving on in the plot. You will get a cinematic prompting you to head down into a cave. This is the Cavern of the Stolen Fayth.
Proceed in and make your way to the back of the cavern. You will meet a Summoner who will summon Yojimbo. You must fight him, but he can be defeated quickly with Aeon overdrives.
Make sure you have at least 250,000 gil before stepping on the teleport pad. Why? Because you need to bid for Yojimbo's loyalty, and he doesn't come cheap!
Select the third option (to defeat the strongest opponents), when he asks you what you would have him do.
Bid! To start, he will ask for 250,000 gil, but don't give him that right off! Offer up 125,001. He will refuse, but lower his price.
Keep bidding half plus 1 of his asking price until it reaches 205,000.
Bid 190,000 to 200,000 gil. Higher bids will help later on, but 190,000 will bring him in cheaper now.
Unlike other Aeons, Yojimbo will not attack on command: You must pay him each time you want him to attack. He has four attacks: Daigoro (where his dog attacks), Kozuka (he throws knives), Wakizashi (slashes with his short sword) and Zanmato (THE best attack in the game: it reduces ANY, yes ANY, enemy to 1 HP).
Pay Yojimbo whatever you wish. As long as you give him something, he'll perform one of the first three attacks. There is a complex system of motivation, enemy level, random chance and gil paid that feed into just which attack he does. Unfortunately, getting him to do the Zanmato attack is not only next to impossible, it is LUDICROUSLY expensive! A good rule is to try and give him at least 100 gil each time. This will keep his motivation up and hopefully not burn a hole in your wallet.
Spur Yojimbo towards his Zanmato attack by keeping his overdrive gauge full at all times, paying him at least 1,000 gil per attack and never letting him die. Not exactly easy, huh? You'll have to keep doing this for a while, but eventually, if luck is on your side, you may see him cut down Nemesis, the biggest fiend in the entire game (bigger than the final boss, strangely enough), to 1 HP in a single stroke. Good luck!
Use Yojimbo sparingly. To help ensure that Yojimbo never dies, it may be advisable to keep him out of boss fights, particularly when fighting Seymour and the Guardian atop Mt. Gagazet. Both of these bosses mean either instant death or certain death for Aeons.