Crowns are customization items that appear in Kingdom Hearts II Final Mix, Kingdom Hearts χ and Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage-, which are obtained after completing specific side-missions.
Kingdom Hearts II Final Mix
A crown costume modifications are unlocked by obtaining Proofs. These are equipped on Sora automatically and are present at all times, except during cutscenes and while Sora is in a world-specific costume. There are several types of crowns; possessing one Proof grants Sora a copper crown, two a silver crown, and all three a gold crown.
Kingdom Hearts χ
In Kingdom Hearts χ, crown accessory items are monthly ranking rewards for events and the Dark Corridor Trials, they are named after the zodiac. Four variants are available: Gold, Silver, Bronze and finally "Charm", which is actually a hair decoration and not a hat like the rest.
Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage-
Bronze, silver, and gold tiaras can be earned as part of the Wardrobe system.