S2·E2 Transient

Yayoi's sister, Sumire is missing. She is constantly hearing the sound of bubbles from underwater. She after a while begins to realize her sister is dead. But because she doesn't know the identity of the killer, she can't get onto the website to get her sister's revenge. She blames herself for Sumire's death because Sumire vanished on a day Yayoi didn't walk with her. Yayoi falls into a fountain and has a vision of what happened to her sister. Sumire was kidnapped, raped, killed, and dumped in a lake, which explains the bubbles. Yayoi knows the identity of the killer and sends him to Hell for Sumire.