You’ll find Anayama just down and to the left of the Ashina Outskirts - Stairway Idol and upon speaking to him, you’ll be able to pay 50 Sen to gain information. Rest again to gain access to his shop along with purchasing more information for 100 Sen. Once you leave him, he’ll ask if you can find out what the Samurai in Ashina Castle are in short supply of.
Second Shop Update
Once you reach Ashina Castle, head to the Old Grave Idol and head to the bridge to your left by swimming under it to find yourself behind the two Samurai. Take the moment to Eavesdrop here to learn that they are in short supply of Rice. Take this information back to Anayama to have him upgrade the shop a second time. Before you leave, he’ll ask if you can find him a bodyguard, someone who is large and would have no issue with looting the dead.
Third Shop Update
When you reach Senpou Temple, you’ll find a large man sitting by the tree to the right as you exit the first room here. You’ll learn that he’s looking for a White Pinwheel and you’ll find that along with a Red and White Pinwheel during your time at Senpou Temple. When you return to Kotaro, hand him the Red and White Pinwheel and you’ll then be given a choice of locations to send him, choose to send him to Anayama.
Return to Anayama to find the two of them together and the shop upgrade a third time.
Fourth Shop Update
Once you defeat the Corrupted Monk at the Fountainhead Palace, you’ll obtain the Dragon’s Tally Board which will unlock a further three items from Anayama’s shop. At this point, be sure to purchase everything you need or want from his shop before completing the Fountainhead Palace where you’ll lose access to it.
During the Final Visit to Ashina Outskirts
During the final area of the game, when the Central Forces have set up a blockade in the Outskirts, head to the other side of where Anayama usually sells his wares. You’ll find Kotaro dead and Anayama next to him, also not long for this world. You’ll be given the choice to purchase the Promissory Note for One Sen, be sure to buy it as it’s your only opportunity on that playthrough to get it. This will reduce the cost of all items from any merchant by 10%, perfect for a new game+.