My guess is that Shion used to have some kind of relationship with mc's grandfather. He appeared after mc threw the glass jar into the sea as he promised his grandfather and showed a mysterious emotion when he knew mc's grandfather was dead. Btw, Yonehara-san is so cute (=・ω・=) I think he has something to do with the time when mc lost his memory. In the raw we got to see him cry and his tear is kinda like the mermaid's tear that mc has been holding on to all this time. We also known that his first love is related to that mysterious mermaid. Maybe that mermaid has saved him when he was drowning.
My guess is that Shion used to have some kind of relationship with mc's grandfather. He appeared after mc threw the glass jar into the sea as he promised his grandfather and showed a mysterious emotion when he knew mc's grandfather was dead.
Btw, Yonehara-san is so cute (=・ω・=) I think he has something to do with the time when mc lost his memory. In the raw we got to see him cry and his tear is kinda like the mermaid's tear that mc has been holding on to all this time. We also known that his first love is related to that mysterious mermaid. Maybe that mermaid has saved him when he was drowning.