Which resources do I need to upgrade a dragon's Habit?

To raise a Habit by one level you need two items from chests:

• a Breedmark tied to the dragon's breed (Champion / Warrior / Sentinel / Hunter),
• a rarity Core matching the dragon's rarity:
• 🔵 Rare Core → Rare dragons
• 🟣 Epic Core → Epic dragons
• 🟡 Legendary Core → Legendary dragons

An observed cost is 100 Breedmarks + 100 Cores per tier, but that cost rises at higher levels (the exact curve is still to be confirmed).

Cores can also be bought at the Market (with Relic Shards). Since Habits are permanent, these investments never reset.