If you're looking to increase your health ("properly" buffing your character), you need (found after mini-bosses) or Gourd Seeds (to increase your healing flask uses).
Sold by the Pot Noble Harunaga in Hirata Estate for 7 Treasure Carp Scales.
If you are looking for a "piece" to complete a larger set, it's likely one of the three fragments of the Dancing Dragon Mask :
If you're looking to increase your health ("properly" buffing your character), you need (found after mini-bosses) or Gourd Seeds (to increase your healing flask uses).
Sold by the Pot Noble Harunaga in Hirata Estate for 7 Treasure Carp Scales. sekiro-shadows-die-twice-v1-02
If you are looking for a "piece" to complete a larger set, it's likely one of the three fragments of the Dancing Dragon Mask : If you're looking to increase your health ("properly"