Arve Rezzle: Mechanized Fairies (2013)

Original title: アルヴ・レズル -機械仕掛けの妖精たち-

Set in 2022, technology has advanced to the point where the human nervous system can be seamlessly integrated into external networks thanks to nanomachines. People leave their bodies behind in water tanks while traversing through cyberspace. However, a crisis known as "Early Rapture" occurs where the system becomes overloaded, resulting in thousands of consciousnesses being lost in the network. Remu Mikage's sister Shiki is one of them, and in an attempt to look for her he visits her apartment. He encounters Shiki there, who informs him that the person he is speaking to isn't his sister, and has somehow taken over her body.