Opening the Gates
To open the gates behind Iris tower on the fin of Whale Island, you will need to befriend Iris. Talk to Rosetta about moondrop flower then buy moondrop seeds from her. Plant moondrops to the south of the tower and water them every day until they bloom. If you do not harvest the moondrops Iris will invite you up to talk to her. Talk to her every day after 6 p.m.--game time--and give her gifts of moondrop flowers. She will become your friend.
Talk to the holiday merchant, Lute. He will tell you about a special plant that grows on an island in the lake. Go to the island and water the plant growing there. After watering it a few times, it will grow into a beanstalk that you can only climb on holidays. Climb up it on a holiday and talk to Iris. Gelwin will come out and unlock the gate for you.
The whole process may take a while as you'll have to water the moondrops for a week and talk to Iris again and again. Just keep talking to Iris and Lute until they give you the information.