Hotaru is the girl Kumo met while wandering around a small island at the beach
Hotaru has short lavender hair, strajght bangs and three rounded cubes as side bangs, could be resembles braid. She has blue eyes and three thin lines, connected at the end as eyebrowns, 2 visible eyelashes on each eyes. She also has those ever-lasting blush on her cheecks.
Hotaru wears a really light blue tank top and blue shorts with blue strap slippers.
She seems to be friendly and really like dancing as she was so amazed at the dance which lit up the ocean and waited so long just to see it again.
When she first met Kumo, Hotaru said that she has been on the island for a long time, despite it's only a small rock island, she has been there long before Kumo got there. She asked if Kumo is a dancer because he resembles "that guy"- that person she saw long ago, the person whose dance lit up the ocean brilliantly as though it were overjoyed. Moved by such performance, she picked up a sea shell from where that person was standing to perform his dance and propose to Kumo that she wanted to see it again. They later chat for a while until Kumo had to go back after they exchanged their names, while she still chose to remain on the island.
After going to the inn, Kumo finds out that the inn's owner daughter looks exactly like her, whom has been gone for 10 years.
That night, when the dance of the team suprisingly lit up the ocean, Kumo found her standing from affar to watch them. He later picked up a sea shell from where they were standing just like the one she picked up from before.
The next day, while the team found out that the reason the sea lit up was because of water firelies, which seems to be the local's speciality, not some miracle; a sudden guess came in and confirmed herself to be Hotaru-as the inn's owner's daughter. The actual Hotaru is agged and has 2 kids- who has went off and got married 10 years ago, leaved Kumo in a shock, wonder who he really met the day before.
In the end of the episode, the Hotaru Kumo met the day before was still standing on the rock island, gave off a mysterious laugh and faded into thousand of sparkles. Her laughter seems to came from the ocean as it sinks deeper.
Possible theory that she might be the water firelies's spirits because at the very end, a part of the creature was shown where the voice came from as if it's a part of her body.
Water fireflies Hotaru also seems to be able to sense Kumo's injured feet although it has healed properly while he hasn't said anything about it.