Nioh 2 is a pretty difficult game, and there is no harm in seeking help sometimes. If you want to get a stranger to save the day for you, you will need Ochoko Cups. These are used to call in help at a Shrine, or at one of the blue Benevolent Graves that you will find dotted around the world. These blue graves are a new feature and represent other warriors you can summon to help you when the game gets a little too tough.
There are two different graves you can find in the world, excluding the type left behind when you die yourself, and both can summon an AI. The red graves are AI you can fight, but the blue graves are AI you can call in for help. Depending on the level of the warrior the grave summons, it can cost a different amount of Ochoko Cups to summon them. Some of the Benevolent Graves can even summon other players who have placed the grave there to try and assist other people.
While you can find Ochoko Cups while loot chests and corpse around the world, the best way to get them is summoning and defeating Revenants from the red graves. They are guaranteed to drop at least one Ochoko Cup when you kill them.
You can also purchase Ochoko Cups from the Kodama Bazaar, giving you a less violent way to get your hands on them if you are struggling to deal with Revenants.