The protagonist meets this mysterious girl at the island. She always drags around with her a large suitcase.

She would never ever tell what is inside.

Just like Takahara Hairi, she only stays on the island until the end of summer.

With an old map, she wanders around the island looking for something. She said that is a treasure map.

"Let's go. To where the pirate ship dwells."

(Source: Summer Pockets Wiki)