What stays permanent between seasons (Reigns)?

Not everything resets, thankfully. Your permanent capital crosses Reigns and is your real long-term progression:

• Your dragons and their eggs
• Each dragon's Star Rank
• Your dragons' Habits
• Gold (the premium currency, which never resets)
• Dragon Relics, relic fragments, Breedmarks, rarity Cores
• Keys (Dragonbone / Dragonglass / Dragonfire)
• Unlocked Heirloom upgrades plus the Mastery bonus (always active)
• The Lifetime Third Builder, Dragon Strike progress, store bonuses

Golden rule: every bit of Gold, every key, Relic, Breedmark, Core or Heirloom you invest is won forever. Put your effort there first.