1. Get all the NSFW scenes (This includes Fia's, Brownie's and Nyancy's alternate scenes)
2. Get all 8 hints (They're all on Southern Ruby Island, so don't go to Northern yet before completing this
3. Win against Esix first try. (I advise getting Puck's Wrath to avoid Esix' Breath of Terror and The Curse of the Cat-astrophe attacks).
Congrats, you have unlocked the secret ending!