Final Fantasy X Tips

Get Yojimbo to use Zanmato
To get Yojimbo to use Zanmato most of the time, you simply have to pay him a 1/3 of your total gil. Doing so is a guaranteed way of making him use it (well 99% of the time he'll use it).

I believe it also depends slightly on the options that you chose to get him, however it usually works no matter what option you chose.