Purification is one of the four possible endings that can be achieved. Except for the last, every step must be done before defeating the Divine Dragon.
Step 1: stay loyal to Kuro*** Spoiler - click to reveal ***After obtaining the Stone, the Mortal Blade, the Flower and climbing to the top of the castle, you will encounter Owl. Choose "Break the Iron Code, stay loyal to Kuro" and defeat the Great Shinobi.
Step 2: eavesdrop on Kuro*** Spoiler - click to reveal ***Upon defeating Owl, hug against a wall inside the library in Kuro's Room and eavesdrop on him. Rest at the idol and do it two more times.
Step 3: talk to Emma*** Spoiler - click to reveal ***Head upstairs, talk to Emma and choose "I agree. Kuro cannot be allowed to die". Rest at the idol and talk to Emma again.
Travel to Old Grave (Ashina Castle), go through the door to the left and talk to Emma again.
Step 4: eavesdrop on Emma*** Spoiler - click to reveal ***Use the Homeward Idol to travel to the Dilapidated Temple, go to the back of the house and eavesdrop. Talk to Emma, choose "Ask about what you overheard" and "Ask what she is hiding" to receive Father's Bell Charm.
Step 5: acquire Everblossom*** Spoiler - click to reveal ***Interact with the Buddha statue to travel to a different vision of the Hirata State. You'll appear in front of a burning building, where Owl used to be, but this time there is a new mini-boss to defeat. Juzou the Drunkard remains. However, instead of Lady Butterfly, you'll find a more aggressive version of Owl with a slightly different moveset. Defeating him will provide you with Aromatic Flower and a trophy.
Step 6: give Everblossom to Kuro